diff --git a/src/lox/AstPrinter.java b/src/lox/AstPrinter.java
index d22c950..14d15e5 100644
--- a/src/lox/AstPrinter.java
+++ b/src/lox/AstPrinter.java
@@ -16,6 +16,11 @@ public class AstPrinter implements Expr.Visitor<String> {
                 expr.left, expr.right);
     }
 
+    @Override
+    public String visitCallExpr(Expr.Call expr) {
+        return parethesize(expr.paren.lexeme, expr.callee);
+    }
+
     @Override
     public String visitGroupingExpr(Expr.Grouping expr) {
         return parethesize("group", expr.expression);
diff --git a/src/lox/Expr.java b/src/lox/Expr.java
index fb1aedb..5de709e 100644
--- a/src/lox/Expr.java
+++ b/src/lox/Expr.java
@@ -6,6 +6,7 @@ abstract class Expr {
 	interface Visitor<R> {
 		R visitAssignExpr(Assign expr);
 		R visitBinaryExpr(Binary expr);
+		R visitCallExpr(Call expr);
 		R visitGroupingExpr(Grouping expr);
 		R visitLiteralExpr(Literal expr);
 		R visitLogicalExpr(Logical expr);
@@ -42,6 +43,22 @@ abstract class Expr {
 		final Token operator;
 		final Expr right;
 	}
+	static class Call extends Expr {
+		Call(Expr callee, Token paren, List<Expr> arguments) {
+			this.callee = callee;
+			this.paren = paren;
+			this.arguments = arguments;
+		}
+
+		@Override
+		<R> R accept(Visitor<R> visitor) {
+			return visitor.visitCallExpr(this);
+		}
+
+		final Expr callee;
+		final Token paren;
+		final List<Expr> arguments;
+	}
 	static class Grouping extends Expr {
 		Grouping(Expr expression) {
 			this.expression = expression;
diff --git a/src/lox/Interpreter.java b/src/lox/Interpreter.java
index 304a600..85bc28e 100644
--- a/src/lox/Interpreter.java
+++ b/src/lox/Interpreter.java
@@ -1,10 +1,31 @@
 package lox;
 
+import java.util.ArrayList;
 import java.util.List;
 
 public class Interpreter implements Expr.Visitor<Object>,
         Stmt.Visitor<Void> {
-    private Environment environment = new Environment();
+    final Environment globals = new Environment();
+    private Environment environment = globals;
+
+    Interpreter() {
+        globals.define("clock", new LoxCallable() {
+            @Override
+            public int arity() {
+                return 0;
+            }
+
+            @Override
+            public Object call(Interpreter interpreter, List<Object> arguments) {
+                return (double) System.currentTimeMillis() / 1000.0;
+            }
+
+            @Override
+            public String toString() {
+                return "<native fn>";
+            }
+        });
+    }
 
     void interpret(List<Stmt> statements) {
         try {
@@ -69,6 +90,30 @@ public class Interpreter implements Expr.Visitor<Object>,
         }
     }
 
+    @Override
+    public Object visitCallExpr(Expr.Call expr) {
+        Object callee = evaluate(expr.callee);
+
+        List<Object> arguments = new ArrayList<>();
+        for (Expr argument : expr.arguments) {
+            arguments.add(evaluate(argument));
+        }
+
+        if (!(callee instanceof LoxCallable)) {
+            throw new RuntimeError(expr.paren,
+                    "Can only call functions and classes.");
+        }
+
+        LoxCallable function = (LoxCallable) callee;
+        if (arguments.size() != function.arity()) {
+            throw new RuntimeError(expr.paren, "Expected " +
+                    function.arity() + " arguments but got " +
+                    arguments.size() + ".");
+        }
+
+        return function.call(this, arguments);
+    }
+
     @Override
     public Object visitGroupingExpr(Expr.Grouping expr) {
         return evaluate(expr.expression);
@@ -184,6 +229,13 @@ public class Interpreter implements Expr.Visitor<Object>,
         return null;
     }
 
+    @Override
+    public Void visitFunctionStmt(Stmt.Function stmt) {
+        LoxFunction function = new LoxFunction(stmt, environment);
+        environment.define(stmt.name.lexeme, function);
+        return null;
+    }
+
     @Override
     public Void visitIfStmt(Stmt.If stmt) {
         if (isTruthy(evaluate(stmt.condition))) {
@@ -201,6 +253,14 @@ public class Interpreter implements Expr.Visitor<Object>,
         return null;
     }
 
+    @Override
+    public Void visitReturnStmt(Stmt.Return stmt) {
+        Object value = null;
+        if (stmt.value != null) value = evaluate(stmt.value);
+
+        throw new Return(value);
+    }
+
     @Override
     public Void visitVarStmt(Stmt.Var stmt) {
         Object value = null;
diff --git a/src/lox/LoxCallable.java b/src/lox/LoxCallable.java
new file mode 100644
index 0000000..aee3a3d
--- /dev/null
+++ b/src/lox/LoxCallable.java
@@ -0,0 +1,9 @@
+package lox;
+
+import java.util.List;
+
+interface LoxCallable {
+    int arity();
+
+    Object call(Interpreter interpreter, List<Object> arguments);
+}
diff --git a/src/lox/LoxFunction.java b/src/lox/LoxFunction.java
new file mode 100644
index 0000000..9c4ce77
--- /dev/null
+++ b/src/lox/LoxFunction.java
@@ -0,0 +1,39 @@
+package lox;
+
+import java.util.List;
+
+public class LoxFunction implements LoxCallable {
+    private final Stmt.Function declaration;
+    private final Environment closure;
+
+    LoxFunction(Stmt.Function declaration, Environment closure) {
+        this.closure = closure;
+        this.declaration = declaration;
+    }
+
+    @Override
+    public int arity() {
+        return declaration.params.size();
+    }
+
+    @Override
+    public Object call(Interpreter interpreter, List<Object> arguments) {
+        Environment environment = new Environment(closure);
+        for (int i = 0; i < declaration.params.size(); i++) {
+            environment.define(declaration.params.get(i).lexeme,
+                    arguments.get(i));
+        }
+
+        try {
+            interpreter.executeBlock(declaration.body, environment);
+        } catch (Return returnValue) {
+            return returnValue.value;
+        }
+        return null;
+    }
+
+    @Override
+    public String toString() {
+        return "<fn " + declaration.name.lexeme + ">";
+    }
+}
diff --git a/src/lox/Parser.java b/src/lox/Parser.java
index 9fc58ea..f5de1d0 100644
--- a/src/lox/Parser.java
+++ b/src/lox/Parser.java
@@ -32,6 +32,7 @@ public class Parser {
 
     private Stmt declaration() {
         try {
+            if (match(FUN)) return function("function");
             if (match(VAR)) return varDeclaration();
 
             return statement();
@@ -45,6 +46,7 @@ public class Parser {
         if (match(FOR)) return forStatement();
         if (match(IF)) return ifStatement();
         if (match(PRINT)) return printStatement();
+        if (match(RETURN)) return returnStatement();
         if (match(WHILE)) return whileStatement();
         if (match(LEFT_BRACE)) return new Stmt.Block(block());
 
@@ -113,6 +115,17 @@ public class Parser {
         return new Stmt.Print(value);
     }
 
+    private Stmt returnStatement() {
+        Token keyword = previous();
+        Expr value = null;
+        if (!check(SEMICOLON)) {
+            value = expression();
+        }
+
+        consume(SEMICOLON, "Expect ';' after return value.");
+        return new Stmt.Return(keyword, value);
+    }
+
     private Stmt varDeclaration() {
         Token name = consume(IDENTIFIER, "Expect variable name.");
 
@@ -140,6 +153,27 @@ public class Parser {
         return new Stmt.Expression(expr);
     }
 
+    private Stmt.Function function(String kind) {
+        Token name = consume(IDENTIFIER, "Expect " + kind + " name.");
+        consume(LEFT_PAREN, "Expect '(' after " + kind + " name.");
+        List<Token> parameters = new ArrayList<>();
+        if (!check(RIGHT_PAREN)) {
+            do {
+                if (parameters.size() >= 255) {
+                    error(peek(), "Can't have more than 255 parameters.");
+                }
+
+                parameters.add(
+                        consume(IDENTIFIER, "Expect parameter name."));
+            } while (match(COMMA));
+        }
+        consume(RIGHT_PAREN, "Expect ')' after parameters.");
+
+        consume(LEFT_BRACE, "Expect '{' before " + kind + " body.");
+        List<Stmt> body = block();
+        return new Stmt.Function(name, parameters, body);
+    }
+
     private List<Stmt> block() {
         List<Stmt> statements = new ArrayList<>();
 
@@ -248,7 +282,38 @@ public class Parser {
             return new Expr.Unary(operator, right);
         }
 
-        return primary();
+        return call();
+    }
+
+    private Expr finishCall(Expr callee) {
+        List<Expr> arguments = new ArrayList<>();
+        if (!check(RIGHT_PAREN)) {
+            do {
+                if (arguments.size() >= 255) {
+                    error(peek(), "Can't have more than 255 arguments.");
+                }
+                arguments.add(expression());
+            } while (match(COMMA));
+        }
+
+        Token paren = consume(RIGHT_PAREN,
+                "Expect ')' after arguments.");
+
+        return new Expr.Call(callee, paren, arguments);
+    }
+
+    private Expr call() {
+        Expr expr = primary();
+
+        while (true) {
+            if (match(LEFT_PAREN)) {
+                expr = finishCall(expr);
+            } else {
+                break;
+            }
+        }
+
+        return expr;
     }
 
     private Expr primary() {
diff --git a/src/lox/Return.java b/src/lox/Return.java
new file mode 100644
index 0000000..e5570a0
--- /dev/null
+++ b/src/lox/Return.java
@@ -0,0 +1,10 @@
+package lox;
+
+public class Return extends RuntimeException {
+    final Object value;
+
+    Return(Object value) {
+        super(null, null, false, false);
+        this.value = value;
+    }
+}
diff --git a/src/lox/Stmt.java b/src/lox/Stmt.java
index e23ec6c..dbc11fc 100644
--- a/src/lox/Stmt.java
+++ b/src/lox/Stmt.java
@@ -6,8 +6,10 @@ abstract class Stmt {
 	interface Visitor<R> {
 		R visitBlockStmt(Block stmt);
 		R visitExpressionStmt(Expression stmt);
+		R visitFunctionStmt(Function stmt);
 		R visitIfStmt(If stmt);
 		R visitPrintStmt(Print stmt);
+		R visitReturnStmt(Return stmt);
 		R visitVarStmt(Var stmt);
 		R visitWhileStmt(While stmt);
 	}
@@ -35,6 +37,22 @@ abstract class Stmt {
 
 		final Expr expression;
 	}
+	static class Function extends Stmt {
+		Function(Token name, List<Token> params, List<Stmt> body) {
+			this.name = name;
+			this.params = params;
+			this.body = body;
+		}
+
+		@Override
+		<R> R accept(Visitor<R> visitor) {
+			return visitor.visitFunctionStmt(this);
+		}
+
+		final Token name;
+		final List<Token> params;
+		final List<Stmt> body;
+	}
 	static class If extends Stmt {
 		If(Expr condition, Stmt thenBranch, Stmt elseBranch) {
 			this.condition = condition;
@@ -63,6 +81,20 @@ abstract class Stmt {
 
 		final Expr expression;
 	}
+	static class Return extends Stmt {
+		Return(Token keyword, Expr value) {
+			this.keyword = keyword;
+			this.value = value;
+		}
+
+		@Override
+		<R> R accept(Visitor<R> visitor) {
+			return visitor.visitReturnStmt(this);
+		}
+
+		final Token keyword;
+		final Expr value;
+	}
 	static class Var extends Stmt {
 		Var(Token name, Expr initializer) {
 			this.name = name;
diff --git a/src/tool/GenerateAst.java b/src/tool/GenerateAst.java
index 7142b39..eb853ac 100644
--- a/src/tool/GenerateAst.java
+++ b/src/tool/GenerateAst.java
@@ -16,6 +16,7 @@ public class GenerateAst {
         defineAst(outputDir, "Expr", Arrays.asList(
                 "Assign   : Token name, Expr value",
                 "Binary   : Expr left, Token operator, Expr right",
+                "Call     : Expr callee, Token paren, List<Expr> arguments",
                 "Grouping : Expr expression",
                 "Literal  : Object value",
                 "Logical  : Expr left, Token operator, Expr right",
@@ -26,9 +27,12 @@ public class GenerateAst {
         defineAst(outputDir, "Stmt", Arrays.asList(
                 "Block      : List<Stmt> statements",
                 "Expression : Expr expression",
+                "Function   : Token name, List<Token> params," +
+                            " List<Stmt> body",
                 "If         : Expr condition, Stmt thenBranch," +
                             " Stmt elseBranch",
                 "Print      : Expr expression",
+                "Return     : Token keyword, Expr value",
                 "Var        : Token name, Expr initializer",
                 "While      : Expr condition, Stmt body"
         ));