diff --git a/src/pk_compiler.c b/src/pk_compiler.c
index 2b9f4fd..13ee55a 100644
--- a/src/pk_compiler.c
+++ b/src/pk_compiler.c
@@ -218,10 +218,8 @@ typedef enum {
   PREC_LOWEST,
   PREC_LOGICAL_OR,    // or
   PREC_LOGICAL_AND,   // and
-  PREC_LOGICAL_NOT,   // not
   PREC_EQUALITY,      // == !=
-  PREC_IN,            // in
-  PREC_IS,            // is
+  PREC_TEST,          // in is
   PREC_COMPARISION,   // < > <= >=
   PREC_BITWISE_OR,    // |
   PREC_BITWISE_XOR,   // ^
@@ -230,7 +228,7 @@ typedef enum {
   PREC_RANGE,         // ..
   PREC_TERM,          // + -
   PREC_FACTOR,        // * / %
-  PREC_UNARY,         // - ! ~
+  PREC_UNARY,         // - ! ~ not
   PREC_CHAIN_CALL,    // ->
   PREC_CALL,          // ()
   PREC_SUBSCRIPT,     // []
@@ -1192,10 +1190,10 @@ GrammarRule rules[] = {  // Prefix       Infix             Infix Precedence
   /* TK_FUNC       */ { exprFunc,      NULL,             NO_INFIX },
   /* TK_END        */   NO_RULE,
   /* TK_NULL       */ { exprValue,     NULL,             NO_INFIX },
-  /* TK_IN         */ { NULL,          exprBinaryOp,     PREC_IN },
+  /* TK_IN         */ { NULL,          exprBinaryOp,     PREC_TEST },
   /* TK_AND        */ { NULL,          exprAnd,          PREC_LOGICAL_AND },
   /* TK_OR         */ { NULL,          exprOr,           PREC_LOGICAL_OR },
-  /* TK_NOT        */ { exprUnaryOp,   NULL,             PREC_LOGICAL_NOT },
+  /* TK_NOT        */ { exprUnaryOp,   NULL,             PREC_UNARY },
   /* TK_TRUE       */ { exprValue,     NULL,             NO_INFIX },
   /* TK_FALSE      */ { exprValue,     NULL,             NO_INFIX },
   /* TK_DO         */   NO_RULE,
diff --git a/src/pk_core.c b/src/pk_core.c
index 15da694..afb65a7 100644
--- a/src/pk_core.c
+++ b/src/pk_core.c
@@ -1243,9 +1243,7 @@ Var varBitRshift(PKVM* vm, Var v1, Var v2) {
 
 Var varBitNot(PKVM* vm, Var v) {
   int64_t i;
-
   if (!validateInteger(vm, v, &i, "Unary operand")) return VAR_NULL;
-
   return VAR_NUM((double)(~i));
 }
 
@@ -1274,6 +1272,48 @@ bool varLesser(Var v1, Var v2) {
 #undef RIGHT_OPERAND
 #undef UNSUPPORTED_OPERAND_TYPES
 
+bool varContains(PKVM* vm, Var elem, Var container) {
+  if (!IS_OBJ(container)) {
+    VM_SET_ERROR(vm, stringFormat(vm, "'$' is not iterable.",
+                 varTypeName(container)));
+  }
+  Object* obj = AS_OBJ(container);
+
+  switch (obj->type) {
+    case OBJ_STRING: {
+      if (!IS_OBJ_TYPE(elem, OBJ_STRING)) {
+        VM_SET_ERROR(vm, stringFormat(vm, "Expected a string operand."));
+        return false;
+      }
+
+      String* sub = (String*)AS_OBJ(elem);
+      String* str = (String*)AS_OBJ(container);
+      if (sub->length > str->length) return false;
+
+      TODO;
+
+    } break;
+
+    case OBJ_LIST: {
+      List* list = (List*)AS_OBJ(container);
+      for (uint32_t i = 0; i < list->elements.count; i++) {
+        if (isValuesEqual(elem, list->elements.data[i])) return true;
+      }
+      return false;
+    } break;
+
+    case OBJ_MAP:
+    case OBJ_RANGE:
+    case OBJ_SCRIPT:
+    case OBJ_FUNC:
+    case OBJ_FIBER:
+    case OBJ_CLASS:
+    case OBJ_INST:
+      TODO;
+  }
+
+}
+
 // Here we're switching the FNV-1a hash value of the name (cstring). Which is
 // an efficient way than having multiple if (attrib == "name"). From O(n) * k
 // to O(1) where n is the length of the string and k is the number of string
diff --git a/src/pk_core.h b/src/pk_core.h
index 03bfbb1..f5ff55f 100644
--- a/src/pk_core.h
+++ b/src/pk_core.h
@@ -46,6 +46,9 @@ Var varBitNot(PKVM* vm, Var v);               // Returns ~v.
 bool varGreater(Var v1, Var v2); // Returns v1 > v2.
 bool varLesser(Var v1, Var v2);  // Returns v1 < v2.
 
+// Returns elem in container.
+bool varContains(PKVM* vm, Var elem, Var container);
+
 // Returns the attribute named [attrib] on the variable [on].
 Var varGetAttrib(PKVM* vm, Var on, String* attrib);
 
diff --git a/src/pk_vm.c b/src/pk_vm.c
index 4815361..33ce8fc 100644
--- a/src/pk_vm.c
+++ b/src/pk_vm.c
@@ -1477,9 +1477,15 @@ static PkResult runFiber(PKVM* vm, Fiber* fiber) {
     }
 
     OPCODE(IN):
-      // TODO: Implement bool varContaines(vm, on, value);
-      TODO;
-      UNREACHABLE();
+    {
+      // Don't pop yet, we need the reference for gc.
+      Var container = PEEK(-1), elem = PEEK(-2);
+      bool contains = varContains(vm, elem, container);
+      DROP(); DROP(); // container, elem
+      PUSH(VAR_BOOL(contains));
+      CHECK_ERROR();
+      DISPATCH();
+    }
 
     OPCODE(REPL_PRINT):
     {
diff --git a/tests/lang/basics.pk b/tests/lang/basics.pk
index 96bb08a..baf318b 100644
--- a/tests/lang/basics.pk
+++ b/tests/lang/basics.pk
@@ -27,6 +27,11 @@ l1 = [1] + []; assert(l1.length == 1); assert(l1[0] == 1)
 l2 = l1 + [1,2,3]; assert(l2.length == 4); assert(l2 == [1,1,2,3])
 l3 = l2 + l1 + l2; assert(l3 == [1,1,2,3,1,1,1,2,3])
 
+## in tests.
+assert(!('abc' in 'a'))
+assert(42 in [12, 42, 3.14])
+assert(!('a' in ['abc']))
+
 ## Builtin functions tests.
 assert(to_string(42) == '42')