diff --git a/.justfile b/.justfile
index 69346f9..ba4bb2a 100644
--- a/.justfile
+++ b/.justfile
@@ -4,5 +4,8 @@ build:
 run *ARGS:
     zig build run -- {{ARGS}}
 
+run-fast *ARGS:
+    zig build run -Doptimize=ReleaseFast -- {{ARGS}}
+
 test:
     zig build test
diff --git a/README.md b/README.md
index 5c56873..b07000c 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,7 @@ While reading [Crafting Interpreters](https://craftinginterpreters.com/), after
 - [x] 21 - Global Variables
 - [x] 22 - Local Variables
 - [x] 23 - Jumping Back and Forth
-- [ ] 24 - Calls and Functions
+- [x] 24 - Calls and Functions
 - [ ] 25 - Closures
 - [ ] 26 - Garbage Collection
 - [ ] 27 - Classes and Instances
diff --git a/samples/ch24_fib.lox b/samples/ch24_fib.lox
new file mode 100644
index 0000000..d2742e7
--- /dev/null
+++ b/samples/ch24_fib.lox
@@ -0,0 +1,9 @@
+fun fib(n) {
+    if (n < 2) return n;
+    return fib(n - 2) + fib(n - 1);
+}
+
+var start = clock();
+
+print fib(20);
+print clock() - start;
diff --git a/src/compile.zig b/src/compile.zig
index ae0c0b1..98e604f 100644
--- a/src/compile.zig
+++ b/src/compile.zig
@@ -33,9 +33,11 @@ const Precedence = enum {
     Primary,
 };
 
+const ParserFn = *const fn (*Parser, bool) ParsingError!void;
+
 const ParserRule = struct {
-    prefix: ?*const fn (*Parser, bool) ParsingError!void,
-    infix: ?*const fn (*Parser, bool) ParsingError!void,
+    prefix: ?ParserFn,
+    infix: ?ParserFn,
     precedence: Precedence,
 };
 
diff --git a/src/main.zig b/src/main.zig
index 6b28e2f..5912a66 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -63,6 +63,7 @@ pub fn main() !void {
     defer std.process.argsFree(allocator, args);
 
     var vm = VM.new(allocator);
+    vm.init_vm();
     defer vm.destroy();
 
     if (args.len == 1) {
diff --git a/src/object.zig b/src/object.zig
index 69de867..09d8704 100644
--- a/src/object.zig
+++ b/src/object.zig
@@ -3,14 +3,18 @@ const debug = std.debug;
 const Allocator = std.mem.Allocator;
 
 const Chunk = @import("./chunk.zig").Chunk;
+const Value = @import("./values.zig").Value;
 
 const compute_hash = @import("./utils.zig").compute_hash;
 
 pub const ObjType = enum {
     String,
     Function,
+    Native,
 };
 
+pub const NativeFn = *const fn (arg_count: usize, args: []Value) Value;
+
 pub const Obj = struct {
     kind: ObjType,
     allocator: std.mem.Allocator,
@@ -68,6 +72,28 @@ pub const Obj = struct {
         }
     };
 
+    pub const Native = struct {
+        obj: Obj,
+        native: NativeFn,
+
+        pub fn new(allocator: std.mem.Allocator, native: NativeFn) *Native {
+            const obj = Obj{
+                .kind = ObjType.Native,
+                .allocator = allocator,
+            };
+
+            const native_obj = allocator.create(Native) catch unreachable;
+            native_obj.obj = obj;
+            native_obj.native = native;
+
+            return native_obj;
+        }
+
+        pub fn destroy(self: *Native) void {
+            self.obj.allocator.destroy(self);
+        }
+    };
+
     pub fn is_type(self: *Obj, kind: ObjType) bool {
         return self.kind == kind;
     }
@@ -77,7 +103,11 @@ pub const Obj = struct {
     }
 
     pub fn is_function(self: *Obj) bool {
-        return self.is_function(ObjType.Function);
+        return self.is_type(ObjType.Function);
+    }
+
+    pub fn is_native(self: *Obj) bool {
+        return self.is_type(ObjType.Native);
     }
 
     pub fn print(self: *Obj) void {
@@ -94,6 +124,10 @@ pub const Obj = struct {
                     debug.print("<fn {s}>", .{obj.name.?.chars});
                 }
             },
+            ObjType.Native => {
+                // const obj = self.as_native();
+                debug.print("<native fn>", .{});
+            },
         }
     }
 
@@ -107,6 +141,10 @@ pub const Obj = struct {
                 const obj: *Function = @fieldParentPtr("obj", self);
                 obj.destroy();
             },
+            ObjType.Native => {
+                const obj: *Native = @fieldParentPtr("obj", self);
+                obj.destroy();
+            },
         }
     }
 
@@ -119,4 +157,9 @@ pub const Obj = struct {
         std.debug.assert(self.kind == ObjType.Function);
         return @fieldParentPtr("obj", self);
     }
+
+    pub fn as_native(self: *Obj) *Native {
+        std.debug.assert(self.kind == ObjType.Native);
+        return @fieldParentPtr("obj", self);
+    }
 };
diff --git a/src/vm.zig b/src/vm.zig
index abf50f5..08bad7f 100644
--- a/src/vm.zig
+++ b/src/vm.zig
@@ -9,6 +9,7 @@ const OpCode = @import("./opcode.zig").OpCode;
 const Value = @import("./values.zig").Value;
 const Obj = @import("./object.zig").Obj;
 const ObjType = @import("./object.zig").ObjType;
+const NativeFn = @import("./object.zig").NativeFn;
 const Table = @import("./table.zig").Table;
 
 const compile = @import("./compile.zig").compile;
@@ -54,6 +55,10 @@ pub const VM = struct {
         };
     }
 
+    pub fn init_vm(self: *VM) void {
+        self.define_native("clock", clock_native);
+    }
+
     pub fn destroy(self: *VM) void {
         if (constants.DEBUG_PRINT_INTERNAL_STRINGS) {
             self.strings.dump();
@@ -378,6 +383,16 @@ pub const VM = struct {
                 ObjType.Function => {
                     return self.call(callee.as_obj().as_function(), arg_count);
                 },
+                ObjType.Native => {
+                    const native_obj: *Obj.Native = callee.as_obj().as_native();
+                    const value = native_obj.native(
+                        arg_count,
+                        self.stack[self.current_frame().slots_idx - arg_count .. self.current_frame().slots_idx],
+                    );
+                    self.stack_top -= arg_count + 1;
+                    _ = try self.push(value);
+                    return true;
+                },
                 else => {},
             }
         }
@@ -406,4 +421,21 @@ pub const VM = struct {
 
         return true;
     }
+
+    pub fn define_native(self: *VM, name: []const u8, native_fn: NativeFn) void {
+        _ = try self.push(Value.obj_val(&self.copy_string(name).obj));
+        _ = try self.push(Value.obj_val(&Obj.Native.new(self.allocator, native_fn).obj));
+
+        _ = self.globals.set(self.stack[0].as_string(), self.stack[1]);
+
+        _ = self.pop();
+        _ = self.pop();
+    }
+
+    pub fn clock_native(arg_count: usize, args: []Value) Value {
+        const ts = std.time.milliTimestamp();
+        _ = arg_count;
+        _ = args;
+        return Value.number_val(@floatFromInt(ts));
+    }
 };