diff --git a/src/pk_core.c b/src/pk_core.c
index b29d336..4463989 100644
--- a/src/pk_core.c
+++ b/src/pk_core.c
@@ -648,11 +648,34 @@ DEF(coreExit,
 // String functions.
 // -----------------
 
-// TODO: substring.
+DEF(coreStrSub,
+  "str_sub(str:string, pos:num, len:num) -> string\n"
+  "Returns a substring from a given string supplied. In addition, "
+  "the position and length of the substring are provided when this "
+  "function is called. For example: `str_sub(str, pos, len)`.") {
+
+  int64_t len, pos;
+
+  String* str = NULL;
+  if (!validateArgString(vm, 1, &str)) return;
+  if (!validateInteger(vm, ARG(2), &pos, "Argument 2")) return;
+  if (!validateInteger(vm, ARG(3), &len, "Argument 3")) return;
+
+  if (str->length < pos || str->length < len)
+    RET_ERR(newString(vm, "Index out of range."));
+
+  // Edge case, empty string.
+  if (len == 0) RET(VAR_OBJ(newStringLength(vm, "", 0)));
+
+  String* sub_str = newStringLength(vm, str->data + pos, (uint32_t)len);
+
+  RET(VAR_OBJ(sub_str));
+}
 
 DEF(coreStrChr,
   "str_chr(value:num) -> string\n"
   "Returns the ASCII string value of the integer argument.") {
+
   int64_t num;
   if (!validateInteger(vm, ARG(1), &num, "Argument 1")) return;
 
@@ -1123,6 +1146,7 @@ void initializeCore(PKVM* vm) {
   INITIALIZE_BUILTIN_FN("exit",        coreExit,      -1);
 
   // String functions.
+  INITIALIZE_BUILTIN_FN("str_sub",     coreStrSub,     3);
   INITIALIZE_BUILTIN_FN("str_chr",     coreStrChr,     1);
   INITIALIZE_BUILTIN_FN("str_ord",     coreStrOrd,     1);
 
diff --git a/tests/lang/core.pk b/tests/lang/core.pk
index 52d4772..6e07acc 100644
--- a/tests/lang/core.pk
+++ b/tests/lang/core.pk
@@ -30,6 +30,16 @@ def fn(p1, p2, p3) end
 assert(fn.name == "fn")
 assert(fn.arity == 3)
 
+## String functions
+assert(str_sub('c programming', 2, 11) == 'programming')
+assert(str_sub('foobarbaz', 2, 3) == 'oba')
+assert(str_sub('abcdef', 1, 2) == 'bc')
+assert(str_sub('I am a boy', 0, 4) == 'I am')
+assert(str_sub('programming', 2, 0) == '')
+assert(str_sub('foobar', 5, 0) == '')
+assert(str_sub('foobar', 0, 6) == 'foobar')
+assert(str_sub('', 0, 0) == '')
+
 ## range
 r = 1..5
 assert(r.as_list == [1, 2, 3, 4])
@@ -73,3 +83,4 @@ end
 
 # If we got here, that means all test were passed.
 print('All TESTS PASSED')
+