diff --git a/.gitignore b/.gitignore
index 78a0187..a98fadf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -35,4 +35,7 @@ bin/
 # Go workspace file
 go.work
 
+# Go tasks
+.task/
+
 # <--- Go
diff --git a/.task/checksum/astgen b/.task/checksum/astgen
index e2d05de..69cf4bb 100644
--- a/.task/checksum/astgen
+++ b/.task/checksum/astgen
@@ -1 +1 @@
-ef159c739c079a54bc5f6af10e2ea025
+8ad203c83fd99fe71ecaa9da9795cc32
diff --git a/ast/expr.go b/ast/expr.go
index 3a332d4..4400ebf 100644
--- a/ast/expr.go
+++ b/ast/expr.go
@@ -6,6 +6,7 @@ type ExprVisitor[T any] interface {
 	VisitErrorExpr(ee *ErrorExpr) T
 	VisitAssignExpr(ae *AssignExpr) T
 	VisitBinaryExpr(be *BinaryExpr) T
+	VisitCallExpr(ce *CallExpr) T
 	VisitGroupingExpr(ge *GroupingExpr) T
 	VisitLiteralExpr(le *LiteralExpr) T
 	VisitLogicalExpr(le *LogicalExpr) T
@@ -44,6 +45,16 @@ func (be *BinaryExpr) Accept(v ExprVisitor[any]) any {
 	return v.VisitBinaryExpr(be)
 }
 
+type CallExpr struct {
+	Callee Expr
+	Paren token.Token
+	Arguments []Expr
+}
+
+func (ce *CallExpr) Accept(v ExprVisitor[any]) any {
+	return v.VisitCallExpr(ce)
+}
+
 type GroupingExpr struct {
 	Expression Expr
 }
diff --git a/ast/printer.go b/ast/printer.go
index 6d14322..2d89766 100644
--- a/ast/printer.go
+++ b/ast/printer.go
@@ -33,6 +33,25 @@ func (ap *Printer) VisitExpressionStmt(stmt *ExpressionStmt) any {
 	return stmt.Expression.Accept(ap)
 }
 
+func (ap *Printer) VisitFunctionStmt(stmt *FunctionStmt) any {
+	str := "(fun " + stmt.Name.Lexeme + "("
+
+	for i, param := range stmt.Params {
+		if i > 0 {
+			str += ", "
+		}
+		str += param.Lexeme
+
+	}
+
+	str += ")"
+	if stmt.Body != nil {
+		str += " " + ap.bracesizeStmt(stmt.Body...)
+	}
+
+	return str + ")"
+}
+
 func (ap *Printer) VisitPrintStmt(stmt *PrintStmt) any {
 	return ap.parenthesizeExpr("print", stmt.Expression)
 }
@@ -85,6 +104,19 @@ func (ap *Printer) VisitBinaryExpr(expr *BinaryExpr) any {
 	return ap.parenthesizeExpr(expr.Operator.Lexeme, expr.Left, expr.Right)
 }
 
+func (ap *Printer) VisitCallExpr(expr *CallExpr) any {
+	str := expr.Callee.Accept(ap).(string) + "("
+
+	for i, arg := range expr.Arguments {
+		if i > 0 {
+			str += ", "
+		}
+		str += arg.Accept(ap).(string)
+	}
+
+	return str + ")"
+}
+
 func (ap *Printer) VisitGroupingExpr(expr *GroupingExpr) any {
 	return ap.parenthesizeExpr("group", expr.Expression)
 }
diff --git a/ast/printer_test.go b/ast/printer_test.go
index a75b02c..770c9d0 100644
--- a/ast/printer_test.go
+++ b/ast/printer_test.go
@@ -63,6 +63,22 @@ func TestPrintExpr(t *testing.T) {
 			},
 			expected: "(= foo 42)",
 		},
+		{
+			name: "Variable expression",
+			expr: &VariableExpr{
+				Name: token.Token{Type: token.IDENTIFIER, Lexeme: "foo"},
+			},
+			expected: "foo",
+		},
+		{
+			name: "Call expression",
+			expr: &CallExpr{
+				Callee:    &VariableExpr{Name: token.Token{Type: token.IDENTIFIER, Lexeme: "foo"}},
+				Paren:     token.Token{Type: token.LEFT_PAREN, Lexeme: "("},
+				Arguments: []Expr{&LiteralExpr{Value: 1}, &LiteralExpr{Value: 2}},
+			},
+			expected: "foo(1, 2)",
+		},
 	}
 
 	for _, tt := range tests {
@@ -119,6 +135,64 @@ func TestPrintStmts(t *testing.T) {
 			},
 			expected: "(var foo 42)\n",
 		},
+		{
+			name: "Function statement",
+			stmts: []Stmt{
+				&FunctionStmt{
+					Name:   token.Token{Type: token.IDENTIFIER, Lexeme: "foo"},
+					Params: []token.Token{{Type: token.IDENTIFIER, Lexeme: "bar"}},
+					Body: []Stmt{
+						&ExpressionStmt{
+							Expression: &LiteralExpr{Value: 42},
+						},
+					},
+				},
+			},
+			expected: "(fun foo(bar) {\n 42\n})\n",
+		},
+		{
+			name: "Block statement",
+			stmts: []Stmt{
+				&BlockStmt{
+					Statements: []Stmt{
+						&ExpressionStmt{
+							Expression: &LiteralExpr{Value: 1},
+						},
+						&ExpressionStmt{
+							Expression: &LiteralExpr{Value: 2},
+						},
+					},
+				},
+			},
+			expected: "{\n 1\n 2\n}\n",
+		},
+		{
+			name: "If statement",
+			stmts: []Stmt{
+				&IfStmt{
+					Condition: &LiteralExpr{Value: true},
+					ThenBranch: &ExpressionStmt{
+						Expression: &LiteralExpr{Value: 1},
+					},
+					ElseBranch: &ExpressionStmt{
+						Expression: &LiteralExpr{Value: 2},
+					},
+				},
+			},
+			expected: "(if true {\n 1\n} else {\n 2\n})\n",
+		},
+		{
+			name: "While statement",
+			stmts: []Stmt{
+				&WhileStmt{
+					Condition: &LiteralExpr{Value: true},
+					Body: &ExpressionStmt{
+						Expression: &LiteralExpr{Value: 1},
+					},
+				},
+			},
+			expected: "(while true {\n 1\n})\n",
+		},
 	}
 
 	for _, tt := range tests {
@@ -131,58 +205,3 @@ func TestPrintStmts(t *testing.T) {
 		})
 	}
 }
-
-func TestPrintBlockStmt(t *testing.T) {
-	stmt := &BlockStmt{
-		Statements: []Stmt{
-			&ExpressionStmt{
-				Expression: &LiteralExpr{Value: 1},
-			},
-			&ExpressionStmt{
-				Expression: &LiteralExpr{Value: 2},
-			},
-		},
-	}
-
-	printer := NewPrinter()
-	result := printer.PrintStmts([]Stmt{stmt})
-	expected := "{\n 1\n 2\n}\n"
-	if result != expected {
-		t.Errorf("expected %v, got %v", expected, result)
-	}
-}
-
-func TestPrintIfStmt(t *testing.T) {
-	stmt := &IfStmt{
-		Condition: &LiteralExpr{Value: true},
-		ThenBranch: &ExpressionStmt{
-			Expression: &LiteralExpr{Value: 1},
-		},
-		ElseBranch: &ExpressionStmt{
-			Expression: &LiteralExpr{Value: 2},
-		},
-	}
-
-	printer := NewPrinter()
-	result := printer.PrintStmts([]Stmt{stmt})
-	expected := "(if true {\n 1\n} else {\n 2\n})\n"
-	if result != expected {
-		t.Errorf("expected %v, got %v", expected, result)
-	}
-}
-
-func TestPrintWhileStmt(t *testing.T) {
-	stmt := &WhileStmt{
-		Condition: &LiteralExpr{Value: true},
-		Body: &ExpressionStmt{
-			Expression: &LiteralExpr{Value: 1},
-		},
-	}
-
-	printer := NewPrinter()
-	result := printer.PrintStmts([]Stmt{stmt})
-	expected := "(while true {\n 1\n})\n"
-	if result != expected {
-		t.Errorf("expected %v, got %v", expected, result)
-	}
-}
diff --git a/ast/stmt.go b/ast/stmt.go
index e5944f3..2e84bb1 100644
--- a/ast/stmt.go
+++ b/ast/stmt.go
@@ -6,6 +6,7 @@ type StmtVisitor[T any] interface {
 	VisitErrorStmt(es *ErrorStmt) T
 	VisitBlockStmt(bs *BlockStmt) T
 	VisitExpressionStmt(es *ExpressionStmt) T
+	VisitFunctionStmt(fs *FunctionStmt) T
 	VisitIfStmt(is *IfStmt) T
 	VisitPrintStmt(ps *PrintStmt) T
 	VisitVarStmt(vs *VarStmt) T
@@ -40,6 +41,16 @@ func (es *ExpressionStmt) Accept(v StmtVisitor[any]) any {
 	return v.VisitExpressionStmt(es)
 }
 
+type FunctionStmt struct {
+	Name token.Token
+	Params []token.Token
+	Body []Stmt
+}
+
+func (fs *FunctionStmt) Accept(v StmtVisitor[any]) any {
+	return v.VisitFunctionStmt(fs)
+}
+
 type IfStmt struct {
 	Condition Expr
 	ThenBranch Stmt
diff --git a/cmd/astgen/main.go b/cmd/astgen/main.go
index ab529cd..56f4bd9 100644
--- a/cmd/astgen/main.go
+++ b/cmd/astgen/main.go
@@ -25,6 +25,7 @@ func main() {
 		"Error    : Value string",
 		"Assign   : Name token.Token, Value Expr",
 		"Binary   : Left Expr, Operator token.Token, Right Expr",
+		"Call     : Callee Expr, Paren token.Token, Arguments []Expr",
 		"Grouping : Expression Expr",
 		"Literal  : Value any",
 		"Logical  : Left Expr, Operator token.Token, Right Expr",
@@ -36,6 +37,7 @@ func main() {
 		"Error        : Value string",
 		"Block        : Statements []Stmt",
 		"Expression   : Expression Expr",
+		"Function     : Name token.Token, Params []token.Token, Body []Stmt",
 		"If           : Condition Expr, ThenBranch Stmt, ElseBranch Stmt",
 		"Print        : Expression Expr",
 		"Var          : Name token.Token, Initializer Expr",
diff --git a/interpreter/callable.go b/interpreter/callable.go
new file mode 100644
index 0000000..52b4217
--- /dev/null
+++ b/interpreter/callable.go
@@ -0,0 +1,73 @@
+package interpreter
+
+import "golox/ast"
+
+// callable is an interface for callables like functions and methods.
+type callable interface {
+	arity() int
+	call(i *Interpreter, arguments []any) any
+}
+
+// nativeCallable is a struct that implements the callable interface.
+type nativeCallable struct {
+	arityFn func() int
+	callFn  func(interpreter *Interpreter, args []any) any
+}
+
+// newNativeCallable creates a new nativeCallable.
+func newNativeCallable(arityFn func() int, callFn func(interpreter *Interpreter, args []any) any) *nativeCallable {
+	return &nativeCallable{
+		arityFn: arityFn,
+		callFn:  callFn,
+	}
+}
+
+// arity returns the number of arguments the callable expects.
+func (n *nativeCallable) arity() int {
+	return n.arityFn()
+}
+
+// call calls the callable with the given arguments.
+func (n *nativeCallable) call(i *Interpreter, arguments []any) any {
+	return n.callFn(i, arguments)
+}
+
+// String returns a string representation of the callable.
+func (n *nativeCallable) String() string {
+	return "<native fn>"
+}
+
+// function is a struct that implements the callable interface.
+type function struct {
+	declaration *ast.FunctionStmt
+}
+
+// newFunction creates a new function.
+func newFunction(declaration *ast.FunctionStmt) *function {
+	return &function{
+		declaration: declaration,
+	}
+}
+
+// arity returns the number of arguments the function expects.
+func (f *function) arity() int {
+	return len(f.declaration.Params)
+}
+
+// call calls the function with the given arguments.
+func (f *function) call(i *Interpreter, arguments []any) any {
+	env := newEnvironment(i.globals)
+
+	for i, param := range f.declaration.Params {
+		env.define(param.Lexeme, arguments[i])
+	}
+
+	i.executeBlock(f.declaration.Body, env)
+
+	return nil
+}
+
+// String returns a string representation of the function.
+func (f *function) String() string {
+	return "<fn " + f.declaration.Name.Lexeme + ">"
+}
diff --git a/interpreter/interpreter.go b/interpreter/interpreter.go
index 79754db..c4ab809 100644
--- a/interpreter/interpreter.go
+++ b/interpreter/interpreter.go
@@ -6,17 +6,30 @@ import (
 	"golox/errors"
 	"golox/token"
 	"strings"
+	"time"
 )
 
 // Interpreter interprets the AST.
 type Interpreter struct {
 	errLogger errors.Logger
 	env       *environment
+	globals   *environment
 }
 
 // New creates a new Interpreter.
 func New(el errors.Logger) *Interpreter {
-	return &Interpreter{el, newEnvironment(nil)}
+	globals := newEnvironment(nil)
+
+	clockCallable := newNativeCallable(
+		func() int { return 0 },
+		func(interpreter *Interpreter, args []any) any {
+			t := time.Now().UnixNano() / int64(time.Millisecond)
+			return float64(t)
+		})
+
+	globals.define("clock", clockCallable)
+
+	return &Interpreter{el, globals, globals}
 }
 
 // Interpret interprets the AST.
@@ -39,6 +52,13 @@ func (i *Interpreter) VisitExpressionStmt(es *ast.ExpressionStmt) any {
 	return nil
 }
 
+// VisitFunctionStmt visits a function statement.
+func (i *Interpreter) VisitFunctionStmt(fs *ast.FunctionStmt) any {
+	function := newFunction(fs)
+	i.env.define(fs.Name.Lexeme, function)
+	return nil
+}
+
 // VisitIfStmt visits an if statement.
 func (i *Interpreter) VisitIfStmt(is *ast.IfStmt) any {
 	if isTruthy(i.evaluate(is.Condition)) {
@@ -199,6 +219,26 @@ func (i *Interpreter) VisitBinaryExpr(b *ast.BinaryExpr) any {
 	panic(fmt.Sprintf("Unknown binary operator '%s' [line %d]", b.Operator.Lexeme, b.Operator.Line))
 }
 
+// VisitCallExpr visits a CallExpr.
+func (i *Interpreter) VisitCallExpr(c *ast.CallExpr) any {
+	callee := i.evaluate(c.Callee)
+
+	var arguments []any
+	for _, argument := range c.Arguments {
+		arguments = append(arguments, i.evaluate(argument))
+	}
+
+	if f, ok := callee.(callable); ok {
+		if len(arguments) != f.arity() {
+			panic(fmt.Sprintf("Expected %d arguments but got %d [line %d]", f.arity(), len(arguments), c.Paren.Line))
+		}
+
+		return f.call(i, arguments)
+	}
+
+	panic(fmt.Sprintf("Can only call functions and classes [line %d]", c.Paren.Line))
+}
+
 // VisitVariableExpr visits a VariableExpr.
 func (i *Interpreter) VisitVariableExpr(v *ast.VariableExpr) any {
 	return i.env.get(v.Name.Lexeme)
diff --git a/parser/parser.go b/parser/parser.go
index b5d77f4..564b8fe 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -53,8 +53,11 @@ func (p *Parser) Parse() []ast.Stmt {
 	return stmts
 }
 
-// declaration → varDecl | statement ;
+// declaration → funDecl | varDecl | statement ;
 func (p *Parser) declaration() ast.Stmt {
+	if p.match(token.FUN) {
+		return p.function("function")
+	}
 	if p.match(token.VAR) {
 		return p.varDeclaration()
 	}
@@ -236,6 +239,55 @@ func (p *Parser) expressionStatement() ast.Stmt {
 	return &ast.ExpressionStmt{Expression: expr}
 }
 
+// function → "fun" IDENTIFIER "(" parameters? ")" block ;
+func (p *Parser) function(kind string) ast.Stmt {
+	err := p.consume(token.IDENTIFIER, "Expect "+kind+" name.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+	name := p.previous()
+
+	err = p.consume(token.LEFT_PAREN, "Expect '(' after "+kind+" name.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+
+	parameters := []token.Token{}
+	if !p.check(token.RIGHT_PAREN) {
+		for {
+			if len(parameters) >= 255 {
+				p.newErrorExpr(p.peek(), "Cannot have more than 255 parameters.")
+			}
+
+			err = p.consume(token.IDENTIFIER, "Expect parameter name.")
+			if err != nil {
+				return p.fromErrorExpr(err)
+			}
+
+			parameters = append(parameters, p.previous())
+			if !p.match(token.COMMA) {
+				break
+			}
+		}
+	}
+
+	err = p.consume(token.RIGHT_PAREN, "Expect ')' after parameters.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+
+	err = p.consume(token.LEFT_BRACE, "Expect '{' before "+kind+" body.")
+	if err != nil {
+		return p.fromErrorExpr(err)
+	}
+
+	if body, ok := p.blockStatement().(*ast.BlockStmt); ok {
+		return &ast.FunctionStmt{Name: name, Params: parameters, Body: body.Statements}
+	}
+
+	return p.newErrorStmt(p.peek(), "Expected block statement.")
+}
+
 // expression → assignment ;
 func (p *Parser) expression() ast.Expr {
 	return p.assignment()
@@ -337,7 +389,7 @@ func (p *Parser) factor() ast.Expr {
 	return expr
 }
 
-// unary → ( "!" | "-" ) unary | primary ;
+// unary → ( "!" | "-" ) unary | call ;
 func (p *Parser) unary() ast.Expr {
 	if p.match(token.BANG, token.MINUS) {
 		operator := p.previous()
@@ -345,7 +397,47 @@ func (p *Parser) unary() ast.Expr {
 		return &ast.UnaryExpr{Operator: operator, Right: right}
 	}
 
-	return p.primary()
+	return p.call()
+}
+
+// call → primary ( "(" arguments? ")" )* ;
+func (p *Parser) call() ast.Expr {
+	expr := p.primary()
+
+	for {
+		if p.match(token.LEFT_PAREN) {
+			expr = p.finishCall(expr)
+		} else {
+			break
+		}
+	}
+
+	return expr
+}
+
+// finishCall finishes the call expression.
+func (p *Parser) finishCall(callee ast.Expr) ast.Expr {
+	arguments := []ast.Expr{}
+
+	if !p.check(token.RIGHT_PAREN) {
+		for {
+			if len(arguments) >= 255 {
+				p.newErrorExpr(p.peek(), "Cannot have more than 255 arguments.")
+			}
+
+			arguments = append(arguments, p.expression())
+			if !p.match(token.COMMA) {
+				break
+			}
+		}
+	}
+
+	err := p.consume(token.RIGHT_PAREN, "Expect ')' after arguments.")
+	if err != nil {
+		return p.newErrorExpr(p.peek(), err.Value)
+	}
+
+	return &ast.CallExpr{Callee: callee, Paren: p.previous(), Arguments: arguments}
 }
 
 // primary → NUMBER | STRING | "true" | "false" | "nil" | "(" expression ")" | IDENTIFIER;
@@ -439,6 +531,12 @@ func (p *Parser) fromErrorExpr(ee *ast.ErrorExpr) *ast.ErrorStmt {
 	return &ast.ErrorStmt{Value: ee.Value}
 }
 
+// newErrorStmt creates a new ErrorStmt and reports the error.
+func (p *Parser) newErrorStmt(t token.Token, message string) *ast.ErrorStmt {
+	p.errLogger.ErrorAtToken(t, message)
+	return &ast.ErrorStmt{Value: message}
+}
+
 // synchronize synchronizes the parser after an error.
 // It skips tokens until it finds a statement boundary.
 func (p *Parser) synchronize() {
diff --git a/testdata/sayhi.lox b/testdata/sayhi.lox
new file mode 100644
index 0000000..72be92f
--- /dev/null
+++ b/testdata/sayhi.lox
@@ -0,0 +1,13 @@
+var t1 = clock();
+
+fun sayHi(first, last) {
+  print "Hi, " + first + " " + last + "!";
+}
+
+sayHi("Dear", "Reader");
+
+var t2 = clock();
+
+print t1;
+print t2;
+print t1 - t2;