diff --git a/Makefile b/Makefile
index 05ee0f4..724a5e0 100644
--- a/Makefile
+++ b/Makefile
@@ -1,9 +1,9 @@
 
-# ##  Copyright (c) 2020-2021 Thakee Nathees
-# ##  Distributed Under The MIT License
+##  Copyright (c) 2020-2021 Thakee Nathees
+##  Distributed Under The MIT License
 
 CC             = gcc
-CFLAGS         = -fPIC -Wno-int-to-pointer-cast
+CFLAGS         = -fPIC
 DEBUG_CFLAGS   = -D DEBUG -g3 -Og
 RELEASE_CFLAGS = -g -O3
 LDFLAGS        = -lm
diff --git a/README.md b/README.md
index a72cc74..fa16c6d 100644
--- a/README.md
+++ b/README.md
@@ -68,7 +68,7 @@ except for a c99 compatible compiler. It can be compiled with the following comm
 
 #### GCC / MinGw / Clang (alias with gcc)
 ```
-gcc -o pocket cli/*.c src/*.c -Isrc/include -lm -Wno-int-to-pointer-cast
+gcc -o pocket cli/*.c src/*.c -Isrc/include -lm
 ```
 
 #### MSVC
diff --git a/cli/main.c b/cli/main.c
index 0116187..01f497d 100644
--- a/cli/main.c
+++ b/cli/main.c
@@ -130,7 +130,7 @@ PkStringPtr loadScript(PKVM* vm, const char* path) {
 
 int main(int argc, char** argv) {
 
-  const char* usage = "usage: pocket [-c cmd | file]\n";
+  //const char* usage = "usage: pocket [-c cmd | file]\n";
 
   // TODO: implement arg parse, REPL.
 
diff --git a/cli/modules/path.c b/cli/modules/path.c
index 002787f..aa69e30 100644
--- a/cli/modules/path.c
+++ b/cli/modules/path.c
@@ -126,10 +126,10 @@ static void _pathRelpath(PKVM* vm) {
   if (!pkGetArgString(vm, 2, &path)) return;
 
   char abs_from[FILENAME_MAX];
-  size_t len_from = pathAbs(from, abs_from, sizeof(abs_from));
+  pathAbs(from, abs_from, sizeof(abs_from));
 
   char abs_path[FILENAME_MAX];
-  size_t len_path = pathAbs(path, abs_path, sizeof(abs_path));
+  pathAbs(path, abs_path, sizeof(abs_path));
 
   char result[FILENAME_MAX];
   size_t len = cwk_path_get_relative(abs_from, abs_path,
diff --git a/cli/repl.c b/cli/repl.c
index 751725e..61a06a4 100644
--- a/cli/repl.c
+++ b/cli/repl.c
@@ -33,7 +33,7 @@ const char* read_line(uint32_t* length) {
 // Read a line from stdin and returns it without the line ending.
 static void readLine(ByteBuffer* buff) {
   do {
-    char c = fgetc(stdin);
+    char c = (char)fgetc(stdin);
     if (c == EOF || c == '\n') break;
 
     byteBufferWrite(buff, (uint8_t)c);
diff --git a/docs/TODO.txt b/docs/TODO.txt
index d4a4cd2..f6ece8f 100644
--- a/docs/TODO.txt
+++ b/docs/TODO.txt
@@ -6,6 +6,8 @@
 
 // To implement.
 
+- no __file__ for core modules.
+
 - def f(x)
     f(x) ## not tco, unless return f(x)
   end
diff --git a/src/include/pocketlang.h b/src/include/pocketlang.h
index b558428..24fdfd2 100644
--- a/src/include/pocketlang.h
+++ b/src/include/pocketlang.h
@@ -89,6 +89,8 @@ typedef enum {
   PK_SCRIPT,
   PK_FUNCTION,
   PK_FIBER,
+  PK_CLASS,
+  PK_INST,
 } PkVarType;
 
 typedef struct PkStringPtr PkStringPtr;
@@ -387,7 +389,6 @@ PK_PUBLIC PkHandle* pkNewFiber(PKVM* vm, PkHandle* fn);
 //PK_PUBLIC PkVar pkPushBool(PKVM* vm, bool value);
 //PK_PUBLIC PkVar pkPushNumber(PKVM* vm, double value);
 
-
 #ifdef __cplusplus
 } // extern "C"
 #endif
diff --git a/src/pk_common.h b/src/pk_common.h
index a144cfa..3bc3a17 100644
--- a/src/pk_common.h
+++ b/src/pk_common.h
@@ -77,6 +77,14 @@
 /* COMMON MACROS                                                             */
 /*****************************************************************************/
 
+#if defined(__GNUC__)
+  #pragma GCC diagnostic ignored "-Wint-to-pointer-cast"
+  #pragma GCC diagnostic ignored "-Wunused-parameter"
+#elif defined(__clang__)
+  #pragma clang diagnostic ignored "-Wint-to-pointer-cast"
+  #pragma clang diagnostic ignored "-Wunused-parameter"
+#endif
+
 #include <stdio.h> //< Only needed here for ASSERT() macro and for release mode
                    //< TODO; macro use this to print a crash report.
 
diff --git a/src/pk_compiler.c b/src/pk_compiler.c
index 3614eda..f197da3 100644
--- a/src/pk_compiler.c
+++ b/src/pk_compiler.c
@@ -17,9 +17,13 @@
 #define MAX_VARIABLES 256
 
 // The maximum number of functions a script could contain. Also it's limited by
-// it's opcode which is using a single byte value to identify the local.
+// it's opcode which is using a single byte value to identify.
 #define MAX_FUNCTIONS 256
 
+// The maximum number of classes a script could contain. Also it's limited by
+// it's opcode which is using a single byte value to identify.
+#define MAX_CLASSES 255
+
 // The maximum number of names that were used before defined. Its just the size
 // of the Forward buffer of the compiler. Feel free to increase it if it
 // require more.
@@ -107,6 +111,7 @@ typedef enum {
 
   // Keywords.
   TK_MODULE,     // module
+  TK_CLASS,      // class
   TK_FROM,       // from
   TK_IMPORT,     // import
   TK_AS,         // as
@@ -170,6 +175,7 @@ typedef struct {
 // List of keywords mapped into their identifiers.
 static _Keyword _keywords[] = {
   { "module",   6, TK_MODULE   },
+  { "class",    5, TK_CLASS    },
   { "from",     4, TK_FROM     },
   { "import",   6, TK_IMPORT   },
   { "as",       2, TK_AS       },
@@ -973,6 +979,7 @@ typedef enum {
   NAME_LOCAL_VAR,  //< Including parameter.
   NAME_GLOBAL_VAR,
   NAME_FUNCTION,
+  NAME_CLASS,
   NAME_BUILTIN,    //< Native builtin function.
 } NameDefnType;
 
@@ -1025,6 +1032,14 @@ static NameSearchResult compilerSearchName(Compiler* compiler,
     return result;
   }
 
+  // Search through classes.
+  index = scriptGetClass(compiler->script, name, length);
+  if (index != -1) {
+    result.type = NAME_CLASS;
+    result.index = index;
+    return result;
+  }
+
   // Search through functions.
   index = scriptGetFunc(compiler->script, name, length);
   if (index != -1) {
@@ -1140,6 +1155,7 @@ GrammarRule rules[] = {  // Prefix       Infix             Infix Precedence
   /* TK_SRIGHT     */ { NULL,          exprBinaryOp,     PREC_BITWISE_SHIFT },
   /* TK_SLEFT      */ { NULL,          exprBinaryOp,     PREC_BITWISE_SHIFT },
   /* TK_MODULE     */   NO_RULE,
+  /* TK_CLASS      */   NO_RULE,
   /* TK_FROM       */   NO_RULE,
   /* TK_IMPORT     */   NO_RULE,
   /* TK_AS         */   NO_RULE,
@@ -1293,6 +1309,11 @@ static void exprName(Compiler* compiler) {
         emitByte(compiler, result.index);
         break;
 
+      case NAME_CLASS:
+        emitOpcode(compiler, OP_PUSH_TYPE);
+        emitByte(compiler, result.index);
+        break;
+
       case NAME_BUILTIN:
         emitOpcode(compiler, OP_PUSH_BUILTIN_FN);
         emitByte(compiler, result.index);
@@ -1776,6 +1797,19 @@ static void compilerExitBlock(Compiler* compiler) {
   compiler->scope_depth--;
 }
 
+static void compilerPushFunc(Compiler* compiler, Func* fn,
+                             Function* func, int index) {
+  fn->outer_func = compiler->func;
+  fn->ptr = func;
+  fn->depth = compiler->scope_depth;
+  fn->index = index;
+  compiler->func = fn;
+}
+
+static void compilerPopFunc(Compiler* compiler) {
+  compiler->func = compiler->func->outer_func;
+}
+
 /*****************************************************************************/
 /* COMPILING (EMIT BYTECODE)                                                 */
 /*****************************************************************************/
@@ -1864,6 +1898,94 @@ typedef enum {
 static void compileStatement(Compiler* compiler);
 static void compileBlockBody(Compiler* compiler, BlockType type);
 
+// Compile a type and return it's index in the script's types buffer.
+static int compileType(Compiler* compiler) {
+
+  // Consume the name of the type.
+  consume(compiler, TK_NAME, "Expected a type name.");
+  const char* name = compiler->previous.start;
+  int name_len = compiler->previous.length;
+  NameSearchResult result = compilerSearchName(compiler, name, name_len);
+  if (result.type != NAME_NOT_DEFINED) {
+    parseError(compiler, "Name '%.*s' already exists.", name_len, name);
+  }
+
+  // Create a new type.
+  Class* type = newClass(compiler->vm, compiler->script,
+                         name, (uint32_t)name_len);
+  type->ctor->arity = 0;
+
+  // Check count exceeded.
+  int fn_index = (int)compiler->script->functions.count - 1;
+  if (fn_index == MAX_FUNCTIONS) {
+    parseError(compiler, "A script should contain at most %d functions.",
+               MAX_FUNCTIONS);
+  }
+
+  int ty_index = (int)(compiler->script->classes.count - 1);
+  if (ty_index == MAX_CLASSES) {
+    parseError(compiler, "A script should contain at most %d types.",
+               MAX_CLASSES);
+  }
+
+  // Compile the constructor function.
+  ASSERT(compiler->func->ptr == compiler->script->body, OOPS);
+  Func curr_fn;
+  compilerPushFunc(compiler, &curr_fn, type->ctor, fn_index);
+  compilerEnterBlock(compiler);
+
+  // Push an instance on the stack.
+  emitOpcode(compiler, OP_PUSH_INSTANCE);
+  emitByte(compiler, ty_index);
+
+  skipNewLines(compiler);
+  TokenType next = peek(compiler);
+  while (next != TK_END && next != TK_EOF) {
+
+    // Compile field name.
+    consume(compiler, TK_NAME, "Expected a type name.");
+    const char* f_name = compiler->previous.start;
+    int f_len = compiler->previous.length;
+
+    uint32_t f_index = scriptAddName(compiler->script, compiler->vm,
+                                     f_name, f_len);
+
+    // TODO: Add a string compare macro.
+    String* new_name = compiler->script->names.data[f_index];
+    for (uint32_t i = 0; i < type->field_names.count; i++) {
+      String* prev = compiler->script->names.data[type->field_names.data[i]];
+      if (new_name->hash == prev->hash && new_name->length == prev->length &&
+          memcmp(new_name->data, prev->data, prev->length) == 0) {
+        parseError(compiler, "Class field with name '%s' already exists.",
+                   new_name->data);
+      }
+    }
+
+    pkUintBufferWrite(&type->field_names, compiler->vm, f_index);
+
+    // Consume the assignment expression.
+    consume(compiler, TK_EQ, "Expected an assignment after field name.");
+    compileExpression(compiler); // Assigned value.
+    consumeEndStatement(compiler);
+
+    // At this point the stack top would be the expression.
+    emitOpcode(compiler, OP_INST_APPEND);
+
+    skipNewLines(compiler);
+    next = peek(compiler);
+  }
+  consume(compiler, TK_END, "Expected 'end' after type declaration end.");
+
+  compilerExitBlock(compiler);
+
+  // At this point, the stack top would be the constructed instance. Return it.
+  emitFunctionEnd(compiler);
+
+  compilerPopFunc(compiler);
+
+  return -1; // TODO;
+}
+
 // Compile a function and return it's index in the script's function buffer.
 static int compileFunction(Compiler* compiler, FuncType fn_type) {
 
@@ -1892,13 +2014,8 @@ static int compileFunction(Compiler* compiler, FuncType fn_type) {
                MAX_FUNCTIONS);
   }
 
-  Func curr_func;
-  curr_func.outer_func = compiler->func;
-  curr_func.ptr = func;
-  curr_func.depth = compiler->scope_depth;
-  curr_func.index = fn_index;
-
-  compiler->func = &curr_func;
+  Func curr_fn;
+  compilerPushFunc(compiler, &curr_fn, func, fn_index);
 
   int argc = 0;
   compilerEnterBlock(compiler); // Parameter depth.
@@ -1968,7 +2085,8 @@ static int compileFunction(Compiler* compiler, FuncType fn_type) {
   printf("%s", buff.data);
   pkByteBufferClear(&buff, compiler->vm);
 #endif
-  compiler->func = compiler->func->outer_func;
+
+  compilerPopFunc(compiler);
 
   return fn_index;
 }
@@ -2156,6 +2274,7 @@ static int compilerImportName(Compiler* compiler, int line,
       return result.index;
 
     case NAME_FUNCTION:
+    case NAME_CLASS:
     case NAME_BUILTIN:
       parseError(compiler, "Name '%.*s' already exists.", length, name);
       return -1;
@@ -2196,8 +2315,12 @@ static void compilerImportAll(Compiler* compiler, Script* script) {
   ASSERT(script != NULL, OOPS);
   ASSERT(compiler->scope_depth == DEPTH_GLOBAL, OOPS);
 
-  bool done = false;              //< A flag to jump out of the loop.
-  pkUintBuffer* name_buff = NULL; //< The string buffer to iterate through.
+  // Import all types.
+  for (uint32_t i = 0; i < script->classes.count; i++) {
+    uint32_t name_ind = script->classes.data[i]->name;
+    String* name = script->names.data[name_ind];
+    compilerImportSingleEntry(compiler, name->data, name->length);
+  }
 
   // Import all functions.
   for (uint32_t i = 0; i < script->functions.count; i++) {
@@ -2581,7 +2704,10 @@ static void compileTopLevelStatement(Compiler* compiler) {
   // If the statement is call, this will be set to true.
   compiler->is_last_call = false;
 
-  if (match(compiler, TK_NATIVE)) {
+  if (match(compiler, TK_CLASS)) {
+    compileType(compiler);
+
+  } else if (match(compiler, TK_NATIVE)) {
     compileFunction(compiler, FN_NATIVE);
 
   } else if (match(compiler, TK_DEF)) {
@@ -2624,10 +2750,11 @@ PkResult compile(PKVM* vm, Script* script, const char* source,
   ASSERT(script->body != NULL, OOPS);
   pkByteBufferClear(&script->body->fn->opcodes, vm);
 
-  // Remember the count of the globals and functions, If the compilation failed
-  // discard all the globals and functions added by the compilation.
+  // Remember the count of the globals, functions and types, If the compilation
+  // failed discard all the globals and functions added by the compilation.
   uint32_t globals_count = script->globals.count;
   uint32_t functions_count = script->functions.count;
+  uint32_t types_count = script->classes.count;
 
   Func curr_fn;
   curr_fn.depth = DEPTH_SCRIPT;
@@ -2688,6 +2815,7 @@ PkResult compile(PKVM* vm, Script* script, const char* source,
   if (compiler->has_errors) {
     script->globals.count = script->global_names.count = globals_count;
     script->functions.count = functions_count;
+    script->classes.count = types_count;
   }
 
 #if DEBUG_DUMP_COMPILED_CODE
diff --git a/src/pk_core.c b/src/pk_core.c
index 6919fb2..0673091 100644
--- a/src/pk_core.c
+++ b/src/pk_core.c
@@ -815,12 +815,14 @@ DEF(stdLangDisas,
   RET(VAR_OBJ(dump));
 }
 
+#ifdef DEBUG
 DEF(stdLangDebugBreak,
   "debug_break() -> null\n"
   "A debug function for development (will be removed).") {
 
   DEBUG_BREAK();
 }
+#endif
 
 DEF(stdLangWrite,
   "write(...) -> null\n"
@@ -1080,7 +1082,8 @@ Var varAdd(PKVM* vm, Var v1, Var v2) {
       case OBJ_SCRIPT:
       case OBJ_FUNC:
       case OBJ_FIBER:
-      case OBJ_USER:
+      case OBJ_CLASS:
+      case OBJ_INST:
         break;
     }
   }
@@ -1339,6 +1342,7 @@ Var varGetAttrib(PKVM* vm, Var on, String* attrib) {
       // map = { "foo" : 42, "can't access" : 32 }
       // val = map.foo ## 42
       TODO;
+      UNREACHABLE();
     }
 
     case OBJ_RANGE:
@@ -1371,8 +1375,15 @@ Var varGetAttrib(PKVM* vm, Var on, String* attrib) {
     {
       Script* scr = (Script*)obj;
 
+      // Search in types.
+      int index = scriptGetClass(scr, attrib->data, attrib->length);
+      if (index != -1) {
+        ASSERT_INDEX((uint32_t)index, scr->classes.count);
+        return VAR_OBJ(scr->classes.data[index]);
+      }
+
       // Search in functions.
-      int index = scriptGetFunc(scr, attrib->data, attrib->length);
+      index = scriptGetFunc(scr, attrib->data, attrib->length);
       if (index != -1) {
         ASSERT_INDEX((uint32_t)index, scr->functions.count);
         return VAR_OBJ(scr->functions.data[index]);
@@ -1408,8 +1419,38 @@ Var varGetAttrib(PKVM* vm, Var on, String* attrib) {
     }
 
     case OBJ_FIBER:
-    case OBJ_USER:
       TODO;
+      UNREACHABLE();
+
+    case OBJ_CLASS:
+      TODO;
+      UNREACHABLE();
+
+    case OBJ_INST:
+    {
+      Instance* inst = (Instance*)obj;
+      if (inst->is_native) {
+        TODO;
+
+      } else {
+
+        // TODO: Optimize this with binary search.
+        Class* ty = inst->ins->type;
+        for (uint32_t i = 0; i < ty->field_names.count; i++) {
+          ASSERT_INDEX(i, ty->field_names.count);
+          ASSERT_INDEX(ty->field_names.data[i], ty->owner->names.count);
+          String* f_name = ty->owner->names.data[ty->field_names.data[i]];
+          if (f_name->hash == attrib->hash &&
+              f_name->length == attrib->length &&
+              memcmp(f_name->data, attrib->data, attrib->length) == 0) {
+            return inst->ins->fields.data[i];
+          }
+        }
+      }
+
+      ERR_NO_ATTRIB(vm, on, attrib);
+      return VAR_NULL;
+    }
 
     default:
       UNREACHABLE();
@@ -1485,6 +1526,15 @@ do {                                                                          \
         return;
       }
 
+      index = scriptGetClass(scr, attrib->data, attrib->length);
+      if (index != -1) {
+        ASSERT_INDEX((uint32_t)index, scr->classes.count);
+        ASSERT_INDEX(scr->classes.data[index]->name, scr->names.count);
+        String* name = scr->names.data[scr->classes.data[index]->name];
+        ATTRIB_IMMUTABLE(name->data);
+        return;
+      }
+
       ERR_NO_ATTRIB(vm, on, attrib);
       return;
     }
@@ -1499,10 +1549,39 @@ do {                                                                          \
       ERR_NO_ATTRIB(vm, on, attrib);
       return;
 
-    case OBJ_USER:
-      TODO; //ERR_NO_ATTRIB(vm, on, attrib);
+    case OBJ_CLASS:
+      ERR_NO_ATTRIB(vm, on, attrib);
       return;
 
+    case OBJ_INST:
+    {
+      Instance* inst = (Instance*)obj;
+      if (inst->is_native) {
+        TODO;
+        return;
+
+      } else {
+
+        // TODO: Optimize this with binary search.
+        Class* ty = inst->ins->type;
+        for (uint32_t i = 0; i < ty->field_names.count; i++) {
+          ASSERT_INDEX(i, ty->field_names.count);
+          ASSERT_INDEX(ty->field_names.data[i], ty->owner->names.count);
+          String* f_name = ty->owner->names.data[ty->field_names.data[i]];
+          if (f_name->hash == attrib->hash &&
+            f_name->length == attrib->length &&
+            memcmp(f_name->data, attrib->data, attrib->length) == 0) {
+            inst->ins->fields.data[i] = value;
+            return;
+          }
+        }
+        ERR_NO_ATTRIB(vm, on, attrib);
+        return;
+      }
+
+      UNREACHABLE();
+    }
+
     default:
       UNREACHABLE();
   }
@@ -1574,8 +1653,11 @@ Var varGetSubscript(PKVM* vm, Var on, Var key) {
     case OBJ_SCRIPT:
     case OBJ_FUNC:
     case OBJ_FIBER:
-    case OBJ_USER:
+    case OBJ_CLASS:
+    case OBJ_INST:
       TODO;
+      UNREACHABLE();
+
     default:
       UNREACHABLE();
   }
@@ -1621,8 +1703,11 @@ void varsetSubscript(PKVM* vm, Var on, Var key, Var value) {
     case OBJ_SCRIPT:
     case OBJ_FUNC:
     case OBJ_FIBER:
-    case OBJ_USER:
+    case OBJ_CLASS:
+    case OBJ_INST:
       TODO;
+      UNREACHABLE();
+
     default:
       UNREACHABLE();
   }
diff --git a/src/pk_debug.c b/src/pk_debug.c
index 40ffd1b..816a8c7 100644
--- a/src/pk_debug.c
+++ b/src/pk_debug.c
@@ -96,7 +96,7 @@ void dumpFunctionCode(PKVM* vm, Function* func, pkByteBuffer* buff) {
     const char* op_name = op_names[opcodes[i]];
     uint32_t op_length = (uint32_t)strlen(op_name);
     pkByteBufferAddString(buff, vm, op_name, op_length);
-    for (uint32_t i = 0; i < 16 - op_length; i++) { // Padding.
+    for (uint32_t j = 0; j < 16 - op_length; j++) { // Padding.
       ADD_CHAR(vm, buff, ' ');
     }
 
@@ -124,10 +124,26 @@ void dumpFunctionCode(PKVM* vm, Function* func, pkByteBuffer* buff) {
         NO_ARGS();
         break;
 
-      case OP_PUSH_LIST:   SHORT_ARG(); break;
-      case OP_PUSH_MAP:    NO_ARGS();   break;
-      case OP_LIST_APPEND: NO_ARGS();   break;
-      case OP_MAP_INSERT:  NO_ARGS();   break;
+      case OP_PUSH_LIST:     SHORT_ARG(); break;
+      case OP_PUSH_INSTANCE:
+      {
+        int ty_index = READ_BYTE();
+        ASSERT_INDEX((uint32_t)ty_index, func->owner->classes.count);
+        uint32_t name_ind = func->owner->classes.data[ty_index]->name;
+        ASSERT_INDEX(name_ind, func->owner->names.count);
+        String* ty_name = func->owner->names.data[name_ind];
+
+        // Prints: %5d [Ty:%s]\n
+        ADD_INTEGER(vm, buff, ty_index, INT_WIDTH);
+        pkByteBufferAddString(buff, vm, STR_AND_LEN(" [Ty:"));
+        pkByteBufferAddString(buff, vm, ty_name->data, ty_name->length);
+        pkByteBufferAddString(buff, vm, STR_AND_LEN("]\n"));
+        break;
+      }
+      case OP_PUSH_MAP:      NO_ARGS();   break;
+      case OP_LIST_APPEND:   NO_ARGS();   break;
+      case OP_MAP_INSERT:    NO_ARGS();   break;
+      case OP_INST_APPEND:   NO_ARGS();   break;
 
       case OP_PUSH_LOCAL_0:
       case OP_PUSH_LOCAL_1:
@@ -148,7 +164,7 @@ void dumpFunctionCode(PKVM* vm, Function* func, pkByteBuffer* buff) {
 
         } else {
           arg = (int)(op - OP_PUSH_LOCAL_0);
-          for (int i = 0; i < INT_WIDTH; i++) ADD_CHAR(vm, buff, ' ');
+          for (int j = 0; j < INT_WIDTH; j++) ADD_CHAR(vm, buff, ' ');
         }
 
         if (arg < func->arity) {
@@ -181,7 +197,7 @@ void dumpFunctionCode(PKVM* vm, Function* func, pkByteBuffer* buff) {
 
         } else {
           arg = (int)(op - OP_STORE_LOCAL_0);
-          for (int i = 0; i < INT_WIDTH; i++) ADD_CHAR(vm, buff, ' ');
+          for (int j = 0; j < INT_WIDTH; j++) ADD_CHAR(vm, buff, ' ');
         }
 
         if (arg < func->arity) {
@@ -224,6 +240,23 @@ void dumpFunctionCode(PKVM* vm, Function* func, pkByteBuffer* buff) {
         break;
       }
 
+      case OP_PUSH_TYPE:
+      {
+        int ty_index = READ_BYTE();
+        ASSERT_INDEX((uint32_t)ty_index, func->owner->classes.count);
+        uint32_t name_ind = func->owner->classes.data[ty_index]->name;
+        ASSERT_INDEX(name_ind, func->owner->names.count);
+        String* ty_name = func->owner->names.data[name_ind];
+
+        // Prints: %5d [Ty:%s]\n
+        ADD_INTEGER(vm, buff, ty_index, INT_WIDTH);
+        pkByteBufferAddString(buff, vm, STR_AND_LEN(" [Ty:"));
+        pkByteBufferAddString(buff, vm, ty_name->data, ty_name->length);
+        pkByteBufferAddString(buff, vm, STR_AND_LEN("]\n"));
+
+        break;
+      }
+
       case OP_PUSH_BUILTIN_FN:
       {
         int index = READ_BYTE();
diff --git a/src/pk_opcodes.h b/src/pk_opcodes.h
index f4c7a17..9f60995 100644
--- a/src/pk_opcodes.h
+++ b/src/pk_opcodes.h
@@ -39,6 +39,10 @@ OPCODE(PUSH_LIST, 2, 1)
 // Push a new map to construct from literal.
 OPCODE(PUSH_MAP, 0, 1)
 
+// Push a new instance to the stack.
+// param: 1 byte index.
+OPCODE(PUSH_INSTANCE, 1, 1)
+
 // Pop the value on the stack the next stack top would be a list. Append the
 // value to the list. Used in literal array construction.
 OPCODE(LIST_APPEND, 0, -1)
@@ -47,6 +51,10 @@ OPCODE(LIST_APPEND, 0, -1)
 // Insert the key value pairs to the map. Used in literal map construction.
 OPCODE(MAP_INSERT, 0, -2)
 
+// Pop the value on the stack, the next stack top would be an instance. Append
+// the value to the instance. Used in instance construction.
+OPCODE(INST_APPEND, 0, -1)
+
 // Push stack local on top of the stack. Locals at 0 to 8 marked explicitly
 // since it's performance critical.
 // params: PUSH_LOCAL_N -> 1 byte count value.
@@ -89,6 +97,10 @@ OPCODE(STORE_GLOBAL, 1, 0)
 // params: 1 byte index.
 OPCODE(PUSH_FN, 1, 1)
 
+// Push the script's type on the stack.
+// params: 1 byte index
+OPCODE(PUSH_TYPE, 1, 1)
+
 // Push a built in function.
 // params: 1 bytes index.
 OPCODE(PUSH_BUILTIN_FN, 1, 1)
diff --git a/src/pk_var.c b/src/pk_var.c
index 78de015..e0781ae 100644
--- a/src/pk_var.c
+++ b/src/pk_var.c
@@ -34,7 +34,8 @@ PkVarType pkGetValueType(const PkVar value) {
     case OBJ_SCRIPT: return PK_SCRIPT;
     case OBJ_FUNC:   return PK_FUNCTION;
     case OBJ_FIBER:  return PK_FIBER;
-    case OBJ_USER:   TODO; break;
+    case OBJ_CLASS:  return PK_CLASS;
+    case OBJ_INST:   return PK_INST;
   }
 
   UNREACHABLE();
@@ -83,6 +84,7 @@ DEFINE_BUFFER(Byte, uint8_t)
 DEFINE_BUFFER(Var, Var)
 DEFINE_BUFFER(String, String*)
 DEFINE_BUFFER(Function, Function*)
+DEFINE_BUFFER(Class, Class*)
 
 void pkByteBufferAddString(pkByteBuffer* self, PKVM* vm, const char* str,
                            uint32_t length) {
@@ -138,6 +140,7 @@ void markVarBuffer(PKVM* vm, pkVarBuffer* self) {
 
 MARK_OBJ_BUFFER(String)
 MARK_OBJ_BUFFER(Function)
+MARK_OBJ_BUFFER(Class)
 
 #undef MARK_OBJ_BUFFER
 
@@ -192,6 +195,9 @@ static void popMarkedObjectsInternal(Object* obj, PKVM* vm) {
       markFunctionBuffer(vm, &scr->functions);
       vm->bytes_allocated += sizeof(Function*) * scr->functions.capacity;
 
+      markClassBuffer(vm, &scr->classes);
+      vm->bytes_allocated += sizeof(Class*) * scr->classes.count;
+
       markStringBuffer(vm, &scr->names);
       vm->bytes_allocated += sizeof(String*) * scr->names.capacity;
 
@@ -207,8 +213,10 @@ static void popMarkedObjectsInternal(Object* obj, PKVM* vm) {
 
       if (!func->is_native) {
         Fn* fn = func->fn;
+        vm->bytes_allocated += sizeof(Fn);
+
         vm->bytes_allocated += sizeof(uint8_t)* fn->opcodes.capacity;
-        vm->bytes_allocated += sizeof(int) * fn->oplines.capacity;
+        vm->bytes_allocated += sizeof(uint32_t) * fn->oplines.capacity;
       }
     } break;
 
@@ -237,9 +245,24 @@ static void popMarkedObjectsInternal(Object* obj, PKVM* vm) {
 
     } break;
 
-    case OBJ_USER:
-      TODO;
-      break;
+    case OBJ_CLASS:
+    {
+      Class* type = (Class*)obj;
+      vm->bytes_allocated += sizeof(Class);
+      markObject(vm, &type->owner->_super);
+      markObject(vm, &type->ctor->_super);
+      vm->bytes_allocated += sizeof(uint32_t) * type->field_names.capacity;
+    } break;
+
+    case OBJ_INST:
+    {
+      Instance* inst = (Instance*)obj;
+      if (!inst->is_native) {
+        Inst* ins = inst->ins;
+        vm->bytes_allocated += sizeof(Inst);
+        vm->bytes_allocated += sizeof(Var*) * ins->fields.capacity;
+      }
+    } break;
   }
 }
 
@@ -329,6 +352,7 @@ Script* newScript(PKVM* vm, String* path) {
   pkUintBufferInit(&script->global_names);
   pkVarBufferInit(&script->literals);
   pkFunctionBufferInit(&script->functions);
+  pkClassBufferInit(&script->classes);
   pkStringBufferInit(&script->names);
 
   vmPushTempRef(vm, &script->_super);
@@ -434,6 +458,59 @@ Fiber* newFiber(PKVM* vm, Function* fn) {
   return fiber;
 }
 
+Class* newClass(PKVM* vm, Script* scr, const char* name, uint32_t length) {
+  Class* type = ALLOCATE(vm, Class);
+  varInitObject(&type->_super, vm, OBJ_CLASS);
+
+  vmPushTempRef(vm, &type->_super); // type.
+
+  pkClassBufferWrite(&scr->classes, vm, type);
+  type->owner = scr;
+  type->name = scriptAddName(scr, vm, name, length);
+  pkUintBufferInit(&type->field_names);
+
+  // Can't use '$' in string format. (TODO)
+  String* ty_name = scr->names.data[type->name];
+  String* dollar = newStringLength(vm, "$", 1);
+  vmPushTempRef(vm, &dollar->_super); // dollar
+  String* ctor_name = stringFormat(vm, "@(Ctor:@)", dollar, ty_name);
+  vmPopTempRef(vm); // dollar
+
+  // Constructor.
+  vmPushTempRef(vm, &ctor_name->_super); // ctor_name
+  type->ctor = newFunction(vm, ctor_name->data, ctor_name->length,
+                           scr, false, NULL);
+  vmPopTempRef(vm); // ctor_name
+
+  vmPopTempRef(vm); // type.
+  return type;
+}
+
+Instance* newInstance(PKVM* vm, Class* ty, bool initialize) {
+
+  Instance* inst = ALLOCATE(vm, Instance);
+  varInitObject(&inst->_super, vm, OBJ_INST);
+
+  vmPushTempRef(vm, &inst->_super); // inst.
+
+  ASSERT(ty->name < ty->owner->names.count, OOPS);
+  inst->name = ty->owner->names.data[ty->name]->data;
+  inst->is_native = false;
+
+  Inst* ins = ALLOCATE(vm, Inst);
+  inst->ins = ins;
+  ins->type = ty;
+  pkVarBufferInit(&ins->fields);
+
+  if (initialize && ty->field_names.count != 0) {
+    pkVarBufferFill(&ins->fields, vm, VAR_NULL, ty->field_names.count);
+  }
+
+  vmPopTempRef(vm); // inst.
+
+  return inst;
+}
+
 List* rangeAsList(PKVM* vm, Range* self) {
   List* list;
   if (self->from < self->to) {
@@ -678,8 +755,10 @@ static uint32_t _hashObject(Object* obj) {
     case OBJ_SCRIPT:
     case OBJ_FUNC:
     case OBJ_FIBER:
-    case OBJ_USER:
+    case OBJ_CLASS:
+    case OBJ_INST:
       TODO;
+      UNREACHABLE();
 
     default:
     L_unhashable:
@@ -898,6 +977,7 @@ void freeObject(PKVM* vm, Object* self) {
       pkUintBufferClear(&scr->global_names, vm);
       pkVarBufferClear(&scr->literals, vm);
       pkFunctionBufferClear(&scr->functions, vm);
+      pkClassBufferClear(&scr->classes, vm);
       pkStringBufferClear(&scr->names, vm);
     } break;
 
@@ -906,6 +986,7 @@ void freeObject(PKVM* vm, Object* self) {
       if (!func->is_native) {
         pkByteBufferClear(&func->fn->opcodes, vm);
         pkUintBufferClear(&func->fn->oplines, vm);
+        DEALLOCATE(vm, func->fn);
       }
     } break;
 
@@ -915,9 +996,26 @@ void freeObject(PKVM* vm, Object* self) {
       DEALLOCATE(vm, fiber->frames);
     } break;
 
-    case OBJ_USER:
-      TODO; // Remove OBJ_USER.
+    case OBJ_CLASS: {
+      Class* type = (Class*)self;
+      pkUintBufferClear(&type->field_names, vm);
+    } break;
+
+    case OBJ_INST:
+    {
+      Instance* inst = (Instance*)self;
+
+      if (inst->is_native) {
+        TODO;
+
+      } else {
+        Inst* ins = inst->ins;
+        pkVarBufferClear(&ins->fields, vm);
+        DEALLOCATE(vm, ins);
+      }
+
       break;
+    }
   }
 
   DEALLOCATE(vm, self);
@@ -943,23 +1041,36 @@ uint32_t scriptAddName(Script* self, PKVM* vm, const char* name,
   return self->names.count - 1;
 }
 
-uint32_t scriptGetFunc(Script* script, const char* name, uint32_t length) {
-  for (uint32_t i = 0; i < script->functions.count; i++) {
-    const char* fn_name = script->functions.data[i]->name;
-    uint32_t fn_length = (uint32_t)strlen(fn_name);
-    if (fn_length == length && strncmp(fn_name, name, length) == 0) {
-      return i;
+int scriptGetClass(Script* script, const char* name, uint32_t length) {
+  for (uint32_t i = 0; i < script->classes.count; i++) {
+    uint32_t name_ind = script->classes.data[i]->name;
+    ASSERT(name_ind < script->names.count, OOPS);
+    String* ty_name = script->names.data[name_ind];
+    if (ty_name->length == length &&
+        strncmp(ty_name->data, name, length) == 0) {
+      return (int)i;
     }
   }
   return -1;
 }
 
-uint32_t scriptGetGlobals(Script* script, const char* name, uint32_t length) {
+int scriptGetFunc(Script* script, const char* name, uint32_t length) {
+  for (uint32_t i = 0; i < script->functions.count; i++) {
+    const char* fn_name = script->functions.data[i]->name;
+    uint32_t fn_length = (uint32_t)strlen(fn_name);
+    if (fn_length == length && strncmp(fn_name, name, length) == 0) {
+      return (int)i;
+    }
+  }
+  return -1;
+}
+
+int scriptGetGlobals(Script* script, const char* name, uint32_t length) {
   for (uint32_t i = 0; i < script->global_names.count; i++) {
     uint32_t name_index = script->global_names.data[i];
     String* g_name = script->names.data[name_index];
     if (g_name->length == length && strncmp(g_name->data, name, length) == 0) {
-      return i;
+      return (int)i;
     }
   }
   return -1;
@@ -970,9 +1081,9 @@ uint32_t scriptAddGlobal(PKVM* vm, Script* script,
                     Var value) {
 
   // If already exists update the value.
-  uint32_t var_ind = scriptGetGlobals(script, name, length);
+  int var_ind = scriptGetGlobals(script, name, length);
   if (var_ind != -1) {
-    ASSERT(var_ind < script->globals.count, OOPS);
+    ASSERT(var_ind < (int)script->globals.count, OOPS);
     script->globals.data[var_ind] = value;
     return var_ind;
   }
@@ -985,13 +1096,15 @@ uint32_t scriptAddGlobal(PKVM* vm, Script* script,
   return script->globals.count - 1;
 }
 
-// Utility functions //////////////////////////////////////////////////////////
+/*****************************************************************************/
+/* UTILITY FUNCTIONS                                                         */
+/*****************************************************************************/
 
 const char* getPkVarTypeName(PkVarType type) {
   switch (type) {
-    case PK_NULL:     return "null";
-    case PK_BOOL:     return "bool";
-    case PK_NUMBER:   return "number";
+    case PK_NULL:     return "Null";
+    case PK_BOOL:     return "Bool";
+    case PK_NUMBER:   return "Number";
     case PK_STRING:   return "String";
     case PK_LIST:     return "List";
     case PK_MAP:      return "Map";
@@ -999,6 +1112,8 @@ const char* getPkVarTypeName(PkVarType type) {
     case PK_SCRIPT:   return "Script";
     case PK_FUNCTION: return "Function";
     case PK_FIBER:    return "Fiber";
+    case PK_CLASS:    return "Class";
+    case PK_INST:     return "Inst";
   }
 
   UNREACHABLE();
@@ -1013,15 +1128,16 @@ const char* getObjectTypeName(ObjectType type) {
     case OBJ_SCRIPT:  return "Script";
     case OBJ_FUNC:    return "Func";
     case OBJ_FIBER:   return "Fiber";
-    case OBJ_USER:    return "UserObj";
+    case OBJ_CLASS:   return "Class";
+    case OBJ_INST:    return "Inst";
   }
   UNREACHABLE();
 }
 
 const char* varTypeName(Var v) {
-  if (IS_NULL(v)) return "null";
-  if (IS_BOOL(v)) return "bool";
-  if (IS_NUM(v))  return "number";
+  if (IS_NULL(v)) return "Null";
+  if (IS_BOOL(v)) return "Bool";
+  if (IS_NUM(v))  return "Number";
 
   ASSERT(IS_OBJ(v), OOPS);
   Object* obj = AS_OBJ(v);
@@ -1299,9 +1415,40 @@ static void _toStringInternal(PKVM* vm, const Var v, pkByteBuffer* buff,
         return;
       }
 
-      case OBJ_USER: {
-        // TODO:
-        pkByteBufferAddString(buff, vm, "[UserObj]", 9);
+      case OBJ_CLASS: {
+        const Class* ty = (const Class*)obj;
+        pkByteBufferAddString(buff, vm, "[Class:", 7);
+        String* ty_name = ty->owner->names.data[ty->name];
+        pkByteBufferAddString(buff, vm, ty_name->data, ty_name->length);
+        pkByteBufferWrite(buff, vm, ']');
+        return;
+      }
+
+      case OBJ_INST:
+      {
+        const Instance* inst = (const Instance*)obj;
+        pkByteBufferWrite(buff, vm, '[');
+        pkByteBufferAddString(buff, vm, inst->name,
+                              (uint32_t)strlen(inst->name));
+
+        if (!inst->is_native) {
+          const Class* ty = inst->ins->type;
+          const Inst* ins = inst->ins;
+          ASSERT(ins->fields.count == ty->field_names.count, OOPS);
+
+          pkByteBufferWrite(buff, vm, ':');
+          for (uint32_t i = 0; i < ty->field_names.count; i++) {
+            if (i != 0) pkByteBufferWrite(buff, vm, ',');
+
+            pkByteBufferWrite(buff, vm, ' ');
+            String* f_name = ty->owner->names.data[ty->field_names.data[i]];
+            pkByteBufferAddString(buff, vm, f_name->data, f_name->length);
+            pkByteBufferWrite(buff, vm, '=');
+            _toStringInternal(vm, ins->fields.data[i], buff, outer, repr);
+          }
+        }
+
+        pkByteBufferWrite(buff, vm, ']');
         return;
       }
     }
@@ -1351,7 +1498,8 @@ bool toBool(Var v) {
     case OBJ_SCRIPT:
     case OBJ_FUNC:
     case OBJ_FIBER:
-    case OBJ_USER:
+    case OBJ_CLASS:
+    case OBJ_INST:
       return true;
   }
 
diff --git a/src/pk_var.h b/src/pk_var.h
index d970e18..f6fe735 100644
--- a/src/pk_var.h
+++ b/src/pk_var.h
@@ -187,6 +187,8 @@ typedef struct Range Range;
 typedef struct Script Script;
 typedef struct Function Function;
 typedef struct Fiber Fiber;
+typedef struct Class Class;
+typedef struct Instance Instance;
 
 // Declaration of buffer objects of different types.
 DECLARE_BUFFER(Uint, uint32_t)
@@ -194,6 +196,7 @@ DECLARE_BUFFER(Byte, uint8_t)
 DECLARE_BUFFER(Var, Var)
 DECLARE_BUFFER(String, String*)
 DECLARE_BUFFER(Function, Function*)
+DECLARE_BUFFER(Class, Class*)
 
 // Add all the characters to the buffer, byte buffer can also be used as a
 // buffer to write string (like a string stream). Note that this will not
@@ -209,16 +212,14 @@ typedef enum {
   OBJ_SCRIPT,
   OBJ_FUNC,
   OBJ_FIBER,
-  // TODO:
-  OBJ_USER,
+  OBJ_CLASS,
+  OBJ_INST,
 } ObjectType;
 
 // Base struct for all heap allocated objects.
 struct Object {
   ObjectType type;  //< Type of the object in \ref var_Object_Type.
   bool is_marked;   //< Marked when garbage collection's marking phase.
-  //Class* is;      //< The class the object IS. // No OOP in PK.
-
   Object* next;     //< Next object in the heap allocated link list.
 };
 
@@ -269,24 +270,11 @@ struct Script {
   String* module; //< Module name of the script.
   String* path;   //< Path of the script.
 
-  /*
-  names:     ["v1", "fn1", "v2", "fn2", ...]
-               0     1      2     3
-
-  g_names:  [      1,         3 ] <-- function name
-                    0          1   <-- it's index
-
-  globals:  [      fn1,       fn2 ]
-                    0          1
-  */
-
-  // TODO: (maybe) join the function buffer and variable buffers.
-  // and make if possible to override functions. (also have to allow builtin).
-
   pkVarBuffer globals;         //< Script level global variables.
   pkUintBuffer global_names;   //< Name map to index in globals.
 
-  pkFunctionBuffer functions;  //< Script level functions.
+  pkFunctionBuffer functions;  //< Functions of the script.
+  pkClassBuffer classes;       //< Classes of the script.
 
   pkStringBuffer names;        //< Name literals, attribute names, etc.
   pkVarBuffer literals;        //< Script literal constant values.
@@ -305,9 +293,9 @@ typedef struct {
 struct Function {
   Object _super;
 
-  const char* name;      //< Name in the script [owner] or C literal.
-  Script* owner;         //< Owner script of the function.
-  int arity;             //< Number of argument the function expects.
+  const char* name; //< Name in the script [owner] or C literal.
+  Script* owner;    //< Owner script of the function.
+  int arity;        //< Number of argument the function expects.
 
   // Docstring of the function, currently it's just the C string literal
   // pointer, refactor this into String* so that we can support public
@@ -328,10 +316,10 @@ typedef struct {
 } CallFrame;
 
 typedef enum {
-  FIBER_NEW,       //< Fiber haven't started yet.
-  FIBER_RUNNING,   //< Fiber is currently running.
-  FIBER_YIELDED,   //< Yielded fiber, can be resumed.
-  FIBER_DONE,      //< Fiber finished and cannot be resumed.
+  FIBER_NEW,     //< Fiber haven't started yet.
+  FIBER_RUNNING, //< Fiber is currently running.
+  FIBER_YIELDED, //< Yielded fiber, can be resumed.
+  FIBER_DONE,    //< Fiber finished and cannot be resumed.
 } FiberState;
 
 struct Fiber {
@@ -369,6 +357,33 @@ struct Fiber {
   String* error;
 };
 
+struct Class {
+  Object _super;
+
+  Script* owner; //< The script it belongs to.
+  uint32_t name; //< Index of the type's name in the script's name buffer.
+
+  Function* ctor; //< The constructor function.
+  pkUintBuffer field_names; //< Buffer of field names.
+  // TODO: ordered names buffer for binary search.
+};
+
+typedef struct {
+  Class* type;        //< Class this instance belongs to.
+  pkVarBuffer fields; //< Var buffer of the instance.
+} Inst;
+
+struct Instance {
+  Object _super;
+
+  const char* name;  //< Name of the type it belongs to.
+  bool is_native;    //< True if it's a native type instance.
+  union {
+    void* native;  //< C struct pointer. // TODO:
+    Inst* ins;    //< Module instance pointer.
+  };
+};
+
 /*****************************************************************************/
 /* "CONSTRUCTORS"                                                            */
 /*****************************************************************************/
@@ -392,7 +407,7 @@ String* newStringLength(PKVM* vm, const char* text, uint32_t length);
 #else // Macro implementation.
   // Allocate new string using the cstring [text].
   #define newString(vm, text) \
-    newStringLength(vm, text, (text == NULL) ? 0 : (uint32_t)strlen(text))
+    newStringLength(vm, text, (!text) ? 0 : (uint32_t)strlen(text))
 #endif
 
 // Allocate new List and return List*.
@@ -419,6 +434,14 @@ Function* newFunction(PKVM* vm, const char* name, int length, Script* owner,
 // Allocate new Fiber object around the function [fn] and return Fiber*.
 Fiber* newFiber(PKVM* vm, Function* fn);
 
+// Allocate new Class object and return Class* with name [name].
+Class* newClass(PKVM* vm, Script* scr, const char* name, uint32_t length);
+
+// Allocate new instance with of the base [type]. Note that if [initialize] is
+// false, the field value buffer of the instance would be un initialized (ie.
+// the buffer count = 0). Otherwise they'll be set to VAR_NULL.
+Instance* newInstance(PKVM* vm, Class* ty, bool initialize);
+
 /*****************************************************************************/
 /* METHODS                                                                   */
 /*****************************************************************************/
@@ -520,13 +543,17 @@ bool fiberHasError(Fiber* fiber);
 uint32_t scriptAddName(Script* self, PKVM* vm, const char* name,
                        uint32_t length);
 
+// Search for the type name in the script and return it's index in it's
+// [classes] buffer. If not found returns -1.
+int scriptGetClass(Script* script, const char* name, uint32_t length);
+
 // Search for the function name in the script and return it's index in it's
 // [functions] buffer. If not found returns -1.
-uint32_t scriptGetFunc(Script* script, const char* name, uint32_t length);
+int scriptGetFunc(Script* script, const char* name, uint32_t length);
 
 // Search for the global variable name in the script and return it's index in
 // it's [globals] buffer. If not found returns -1.
-uint32_t scriptGetGlobals(Script* script, const char* name, uint32_t length);
+int scriptGetGlobals(Script* script, const char* name, uint32_t length);
 
 // Add a global [value] to the [scrpt] and return its index.
 uint32_t scriptAddGlobal(PKVM* vm, Script* script,
diff --git a/src/pk_vm.c b/src/pk_vm.c
index 94e8eaf..51f93c9 100644
--- a/src/pk_vm.c
+++ b/src/pk_vm.c
@@ -755,6 +755,15 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
       DISPATCH();
     }
 
+    OPCODE(PUSH_INSTANCE):
+    {
+      uint8_t index = READ_BYTE();
+      ASSERT_INDEX(index, script->classes.count);
+      Instance* inst = newInstance(vm, script->classes.data[index], false);
+      PUSH(VAR_OBJ(inst));
+      DISPATCH();
+    }
+
     OPCODE(LIST_APPEND):
     {
       Var elem = PEEK(-1); // Don't pop yet, we need the reference for gc.
@@ -785,6 +794,21 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
       DISPATCH();
     }
 
+    OPCODE(INST_APPEND):
+    {
+      Var value = PEEK(-1); // Don't pop yet, we need the reference for gc.
+      Var inst = PEEK(-2);
+      ASSERT(IS_OBJ_TYPE(inst, OBJ_INST), OOPS);
+
+      Instance* inst_p = (Instance*)AS_OBJ(inst);
+      ASSERT(!inst_p->is_native, OOPS);
+      Inst* ins = inst_p->ins;
+      pkVarBufferWrite(&ins->fields, vm, value);
+      DROP(); // value
+
+      DISPATCH();
+    }
+
     OPCODE(PUSH_LOCAL_0):
     OPCODE(PUSH_LOCAL_1):
     OPCODE(PUSH_LOCAL_2):
@@ -830,7 +854,7 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     OPCODE(PUSH_GLOBAL):
     {
       uint8_t index = READ_BYTE();
-      ASSERT(index < script->globals.count, OOPS);
+      ASSERT_INDEX(index, script->globals.count);
       PUSH(script->globals.data[index]);
       DISPATCH();
     }
@@ -838,7 +862,7 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     OPCODE(STORE_GLOBAL):
     {
       uint8_t index = READ_BYTE();
-      ASSERT(index < script->globals.count, OOPS);
+      ASSERT_INDEX(index, script->globals.count);
       script->globals.data[index] = PEEK(-1);
       DISPATCH();
     }
@@ -846,12 +870,21 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     OPCODE(PUSH_FN):
     {
       uint8_t index = READ_BYTE();
-      ASSERT(index < script->functions.count, OOPS);
+      ASSERT_INDEX(index, script->functions.count);
       Function* fn = script->functions.data[index];
       PUSH(VAR_OBJ(fn));
       DISPATCH();
     }
 
+    OPCODE(PUSH_TYPE):
+    {
+      uint8_t index = READ_BYTE();
+      ASSERT_INDEX(index, script->classes.count);
+      Class* ty = script->classes.data[index];
+      PUSH(VAR_OBJ(ty));
+      DISPATCH();
+    }
+
     OPCODE(PUSH_BUILTIN_FN):
     {
       uint8_t index = READ_BYTE();
@@ -890,64 +923,73 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
       Fiber* call_fiber = vm->fiber;
       Var* callable = call_fiber->sp - argc - 1;
 
+      const Function* fn = NULL;
+
       if (IS_OBJ_TYPE(*callable, OBJ_FUNC)) {
-        const Function* fn = (const Function*)AS_OBJ(*callable);
+        fn = (const Function*)AS_OBJ(*callable);
 
-        // -1 argument means multiple number of args.
-        if (fn->arity != -1 && fn->arity != argc) {
-          char buff[STR_INT_BUFF_SIZE]; sprintf(buff, "%d", fn->arity);
-          String* msg = stringFormat(vm, "Expected exactly $ argument(s).",
-                                     buff);
-          RUNTIME_ERROR(msg);
-        }
-
-        if (fn->is_native) {
-
-          if (fn->native == NULL) {
-            RUNTIME_ERROR(stringFormat(vm,
-              "Native function pointer of $ was NULL.", fn->name));
-          }
-
-          // Update the current frame's ip.
-          UPDATE_FRAME();
-
-          // Next call frame starts here. (including return value).
-          call_fiber->ret = callable;
-          *(call_fiber->ret) = VAR_NULL; //< Set the return value to null.
-
-          fn->native(vm); //< Call the native function.
-
-          // Calling yield() will change vm->fiber to it's caller fiber, which
-          // would be null if we're not running the function with a fiber.
-          if (vm->fiber == NULL) return PK_RESULT_SUCCESS;
-
-          // Load the top frame to vm's execution variables.
-          if (vm->fiber != call_fiber) LOAD_FRAME();
-
-          // Pop function arguments except for the return value.
-          // Don't use 'vm->fiber' because calling fiber_new() and yield()
-          // would change the fiber.
-          call_fiber->sp = call_fiber->ret + 1;
-          CHECK_ERROR();
-
-        } else {
-
-          if (instruction == OP_CALL) {
-            UPDATE_FRAME(); //< Update the current frame's ip.
-            pushCallFrame(vm, fn, callable);
-            LOAD_FRAME();  //< Load the top frame to vm's execution variables.
-
-          } else {
-            ASSERT(instruction == OP_TAIL_CALL, OOPS);
-
-            reuseCallFrame(vm, fn);
-            LOAD_FRAME();  //< Re-load the frame to vm's execution variables.
-          }
-        }
+      } else if (IS_OBJ_TYPE(*callable, OBJ_CLASS)) {
+        fn = (const Function*)((Class*)AS_OBJ(*callable))->ctor;
 
       } else {
         RUNTIME_ERROR(stringFormat(vm, "$ $(@).", "Expected a function in "
-        "call, instead got", varTypeName(*callable), toString(vm, *callable)));
+                      "call, instead got",
+                      varTypeName(*callable), toString(vm, *callable)));
+        DISPATCH();
+      }
+
+      // If we reached here it's a valid callable.
+
+      // -1 argument means multiple number of args.
+      if (fn->arity != -1 && fn->arity != argc) {
+        char buff[STR_INT_BUFF_SIZE]; sprintf(buff, "%d", fn->arity);
+        String* msg = stringFormat(vm, "Expected exactly $ argument(s).",
+                                   buff);
+        RUNTIME_ERROR(msg);
+      }
+
+      if (fn->is_native) {
+
+        if (fn->native == NULL) {
+          RUNTIME_ERROR(stringFormat(vm,
+            "Native function pointer of $ was NULL.", fn->name));
+        }
+
+        // Update the current frame's ip.
+        UPDATE_FRAME();
+
+        // Next call frame starts here. (including return value).
+        call_fiber->ret = callable;
+        *(call_fiber->ret) = VAR_NULL; //< Set the return value to null.
+
+        fn->native(vm); //< Call the native function.
+
+        // Calling yield() will change vm->fiber to it's caller fiber, which
+        // would be null if we're not running the function with a fiber.
+        if (vm->fiber == NULL) return PK_RESULT_SUCCESS;
+
+        // Load the top frame to vm's execution variables.
+        if (vm->fiber != call_fiber) LOAD_FRAME();
+
+        // Pop function arguments except for the return value.
+        // Don't use 'vm->fiber' because calling fiber_new() and yield()
+        // would change the fiber.
+        call_fiber->sp = call_fiber->ret + 1;
+        CHECK_ERROR();
+
+      } else {
+
+        if (instruction == OP_CALL) {
+          UPDATE_FRAME(); //< Update the current frame's ip.
+          pushCallFrame(vm, fn, callable);
+          LOAD_FRAME();  //< Load the top frame to vm's execution variables.
+
+        } else {
+          ASSERT(instruction == OP_TAIL_CALL, OOPS);
+
+          reuseCallFrame(vm, fn);
+          LOAD_FRAME();  //< Re-load the frame to vm's execution variables.
+        }
       }
 
       DISPATCH();
@@ -1051,7 +1093,8 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
         case OBJ_SCRIPT:
         case OBJ_FUNC:
         case OBJ_FIBER:
-        case OBJ_USER:
+        case OBJ_CLASS:
+        case OBJ_INST:
           TODO; break;
         default:
           UNREACHABLE();
@@ -1433,6 +1476,7 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     OPCODE(IN):
       // TODO: Implement bool varContaines(vm, on, value);
       TODO;
+      UNREACHABLE();
 
     OPCODE(REPL_PRINT):
     {
@@ -1447,7 +1491,7 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     }
 
     OPCODE(END):
-      TODO;
+      UNREACHABLE();
       break;
 
     default:
@@ -1455,5 +1499,5 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
 
   }
 
-  return PK_RESULT_SUCCESS;
+  UNREACHABLE(); //return PK_RESULT_SUCCESS;
 }
diff --git a/tests/lang/class.pk b/tests/lang/class.pk
new file mode 100644
index 0000000..93792b9
--- /dev/null
+++ b/tests/lang/class.pk
@@ -0,0 +1,36 @@
+
+## TODO: Implement ctor with va arg to
+## initialize, fields.
+
+class _Vec
+  x = 0
+  y = 0
+end
+
+def Vec(x, y)
+  ret = _Vec()
+  ret.x = x; ret.y = y
+  return ret
+end
+
+def vecAdd(v1, v2)
+  return Vec(v1.x + v2.x,
+             v1.y + v2.y)
+end
+
+v1 = Vec(1, 2); assert(v1.x == 1 and v1.y == 2)
+v2 = Vec(3, 4); assert(v2.x == 3 and v2.y == 4)
+
+v3 = vecAdd(v1, v2)
+assert(v3.x == 4 and v3.y == 6)
+
+class Test
+  fn  = null
+  val = Vec(12, 32)
+end
+
+test = Test()
+test.fn = to_string
+res = test.fn(test.val)
+assert(res == "[_Vec: x=12, y=32]")
+
diff --git a/tests/lang/import.pk b/tests/lang/import.pk
index ce84fa7..be57a4f 100644
--- a/tests/lang/import.pk
+++ b/tests/lang/import.pk
@@ -13,6 +13,10 @@ import "basics.pk" ## will import all
 import "controlflow.pk" as if_test
 from "functions.pk" import fn1, fn2 as f2, fn3
 
+import "class.pk" as class_
+assert(class_.Vec(42, 3.14).y == 3.14)
+(vec = class_._Vec()).x = 'a'; assert(vec.x == 'a')
+
 ## If it has a module name it'll bind to that name.
 import 'import/module.pk'
 assert(module_name.get_module_name() == 'module_name')
diff --git a/tests/tests.py b/tests/tests.py
index 718eb3c..9cf13e0 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -16,6 +16,7 @@ THIS_PATH = abspath(dirname(__file__))
 TEST_SUITE = {
   "Unit Tests": (
     "lang/basics.pk",
+    "lang/class.pk",
     "lang/core.pk",
     "lang/controlflow.pk",
     "lang/fibers.pk",