diff --git a/corelib.wren b/corelib.wren
new file mode 100644
index 00000000..17e83eb7
--- /dev/null
+++ b/corelib.wren
@@ -0,0 +1,23 @@
+// Note: This is converted to a C string literal and inserted into
+// src/wren_core.c using make_corelib.
+class IO {
+  static write(obj) {
+    IO.write__native__(obj.toString)
+    return obj
+  }
+}
+
+class List {
+  toString {
+    var result = "["
+    var i = 0
+    // TODO: Use for loop.
+    while (i < this.count) {
+      if (i > 0) result = result + ", "
+      result = result + this[i].toString
+      i = i + 1
+    }
+    result = result + "]"
+    return result
+  }
+}
diff --git a/make_corelib b/make_corelib
new file mode 100755
index 00000000..fe5098ce
--- /dev/null
+++ b/make_corelib
@@ -0,0 +1,31 @@
+#!/usr/bin/python
+import re
+
+with open("corelib.wren", "r") as f:
+  lines = f.readlines()
+
+# Remove the comment from the top.
+lines.pop(0)
+lines.pop(0)
+
+corelib = ""
+for line in lines:
+  line = line.replace('"', "\\\"")
+  line = line.replace("\n", "\\n\"")
+  if corelib: corelib += "\n"
+  corelib += '"' + line
+
+with open("src/wren_core.c", "r") as f:
+  wren_core = f.read()
+
+# re.sub() will unescape escape sequences, but we want them to stay as escapes
+# in the C string literal.
+corelib = corelib.replace('\\', '\\\\')
+
+corelib = "coreLibSource =\n" + corelib + ";"
+
+PATTERN = re.compile(r'coreLibSource =\n("(.|[\n])*?);')
+wren_core = PATTERN.sub(corelib, wren_core)
+
+with open("src/wren_core.c", "w") as f:
+  f.write(wren_core)
diff --git a/src/wren_core.c b/src/wren_core.c
index e8d62fb2..b31f4285 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -47,6 +47,31 @@
 // to a list has O(1) amortized complexity.
 #define LIST_GROW_FACTOR (2)
 
+// This string literal is generated automatically from corelib.wren using
+// make_corelib. Do not edit here.
+const char* coreLibSource =
+"class IO {\n"
+"  static write(obj) {\n"
+"    IO.write__native__(obj.toString)\n"
+"    return obj\n"
+"  }\n"
+"}\n"
+"\n"
+"class List {\n"
+"  toString {\n"
+"    var result = \"[\"\n"
+"    var i = 0\n"
+"    // TODO: Use for loop.\n"
+"    while (i < this.count) {\n"
+"      if (i > 0) result = result + \", \"\n"
+"      result = result + this[i].toString\n"
+"      i = i + 1\n"
+"    }\n"
+"    result = result + \"]\"\n"
+"    return result\n"
+"  }\n"
+"}\n";
+
 DEF_NATIVE(bool_not)
 {
   return BOOL_VAL(!AS_BOOL(args[0]));
@@ -340,6 +365,11 @@ DEF_NATIVE(object_new)
   return args[0];
 }
 
+DEF_NATIVE(object_toString)
+{
+  return wrenNewString(vm, "<object>", 8);
+}
+
 DEF_NATIVE(object_type)
 {
   return OBJ_VAL(wrenGetClass(vm, args[0]));
@@ -467,6 +497,7 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->objectClass, "== ", object_eqeq);
   NATIVE(vm->objectClass, "!= ", object_bangeq);
   NATIVE(vm->objectClass, "new", object_new);
+  NATIVE(vm->objectClass, "toString", object_toString);
   NATIVE(vm->objectClass, "type", object_type);
 
   // Now we can define Class, which is a subclass of Object, but Object's
@@ -524,15 +555,6 @@ void wrenInitializeCore(WrenVM* vm)
   FIBER_NATIVE(vm->fnClass, "call               ", fn_call15);
   FIBER_NATIVE(vm->fnClass, "call                ", fn_call16);
 
-  vm->listClass = defineClass(vm, "List");
-  NATIVE(vm->listClass, "add ", list_add);
-  NATIVE(vm->listClass, "clear", list_clear);
-  NATIVE(vm->listClass, "count", list_count);
-  NATIVE(vm->listClass, "insert  ", list_insert);
-  NATIVE(vm->listClass, "removeAt ", list_removeAt);
-  NATIVE(vm->listClass, "[ ]", list_subscript);
-  NATIVE(vm->listClass, "[ ]=", list_subscriptSetter);
-
   vm->nullClass = defineClass(vm, "Null");
   NATIVE(vm->nullClass, "toString", null_toString);
 
@@ -565,13 +587,24 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->stringClass, "!= ", string_bangeq);
   NATIVE(vm->stringClass, "[ ]", string_subscript);
 
-  ObjClass* ioClass = defineClass(vm, "IO");
-  NATIVE(ioClass->metaclass, "write ", io_write);
-
   ObjClass* osClass = defineClass(vm, "OS");
   NATIVE(osClass->metaclass, "clock", os_clock);
 
   // TODO: Make this a distinct object type.
   ObjClass* unsupportedClass = wrenNewClass(vm, vm->objectClass, 0);
   vm->unsupported = (Value)wrenNewInstance(vm, unsupportedClass);
+
+  wrenInterpret(vm, coreLibSource);
+
+  vm->listClass = AS_CLASS(findGlobal(vm, "List"));
+  NATIVE(vm->listClass, "add ", list_add);
+  NATIVE(vm->listClass, "clear", list_clear);
+  NATIVE(vm->listClass, "count", list_count);
+  NATIVE(vm->listClass, "insert  ", list_insert);
+  NATIVE(vm->listClass, "removeAt ", list_removeAt);
+  NATIVE(vm->listClass, "[ ]", list_subscript);
+  NATIVE(vm->listClass, "[ ]=", list_subscriptSetter);
+
+  ObjClass* ioClass = AS_CLASS(findGlobal(vm, "IO"));
+  NATIVE(ioClass->metaclass, "write__native__ ", io_write);
 }
diff --git a/test/io/write.wren b/test/io/write.wren
new file mode 100644
index 00000000..10e2b867
--- /dev/null
+++ b/test/io/write.wren
@@ -0,0 +1,12 @@
+class Foo {
+  toString { return "Foo.toString" }
+}
+
+// Calls toString on argument.
+IO.write(new Foo) // expect: Foo.toString
+
+// Returns argument.
+var result = IO.write(123) // expect: 123
+IO.write(result == 123) // expect: true
+
+// TODO: What if toString on argument doesn't return a string?
diff --git a/test/list/to_string.wren b/test/list/to_string.wren
new file mode 100644
index 00000000..1af1282f
--- /dev/null
+++ b/test/list/to_string.wren
@@ -0,0 +1,17 @@
+// Handle empty list.
+IO.write([].toString)             // expect: []
+
+// Does not quote strings.
+IO.write([1, "2", true].toString) // expect: [1, 2, true]
+
+// Nested lists.
+IO.write([1, [2, [3], 4], 5]) // expect: [1, [2, [3], 4], 5]
+
+// Calls toString on elements.
+class Foo {
+  toString { return "Foo.toString" }
+}
+
+IO.write([1, new Foo, 2]) // expect: [1, Foo.toString, 2]
+
+// TODO: Handle lists that contain themselves.
diff --git a/test/object/to_string.wren b/test/object/to_string.wren
new file mode 100644
index 00000000..7b31acc5
--- /dev/null
+++ b/test/object/to_string.wren
@@ -0,0 +1,2 @@
+class Foo {}
+IO.write((new Foo).toString) // expect: <object>