diff --git a/examples/forin.ast b/examples/forin.ast
new file mode 100644
index 0000000..cd7476b
--- /dev/null
+++ b/examples/forin.ast
@@ -0,0 +1,25 @@
+// for-in loop demo
+print("=== array ===")
+let arr = [10, 20, 30]
+for (x in arr) {
+    print(x)
+}
+
+print("=== object keys ===")
+let obj = {name: "Aster", year: 2025}
+let keys = ""
+for (k in obj) {
+    keys = keys + k + " "
+}
+print(keys)
+
+print("=== string chars ===")
+for (c in "Hi") {
+    print(c)
+}
+
+print("=== break ===")
+for (x in [1, 2, 3, 4, 5]) {
+    if (x == 3) { break }
+    print(x)
+}
diff --git a/src/ast/stmt.rs b/src/ast/stmt.rs
index 17f54f3..87b3fb1 100644
--- a/src/ast/stmt.rs
+++ b/src/ast/stmt.rs
@@ -28,7 +28,7 @@ pub enum Stmt {
         body: Box<Stmt>,
     },
 
-    /// for 循环
+    /// for 循环 (C-style)
     For {
         initializer: Option<Box<Stmt>>,
         condition: Option<Expr>,
@@ -36,6 +36,13 @@ pub enum Stmt {
         body: Box<Stmt>,
     },
 
+    /// for-in 循环: for (var in iterable) body
+    ForIn {
+        var_name: String,
+        iterable: Expr,
+        body: Box<Stmt>,
+    },
+
     /// 函数声明
     Function {
         name: String,
diff --git a/src/interpreter/interpreter.rs b/src/interpreter/interpreter.rs
index a979d38..9f54414 100644
--- a/src/interpreter/interpreter.rs
+++ b/src/interpreter/interpreter.rs
@@ -102,6 +102,42 @@ impl Interpreter {
                 }
                 Ok(Signal::None)
             }
+            Stmt::ForIn { var_name, iterable, body } => {
+                let iter_val = self.evaluate(iterable)?;
+                let items: Vec<Value> = match &iter_val {
+                    Value::Array(arr) => arr.borrow().clone(),
+                    Value::Object(obj) => obj.borrow().keys().map(|k| Value::String(k.clone())).collect(),
+                    Value::String(s) => s.chars().map(|c| Value::String(c.to_string())).collect(),
+                    _ => {
+                        return Err(RuntimeError::RuntimeError {
+                            message: "for-in requires an array, object, or string".into(),
+                            token: None,
+                        });
+                    }
+                };
+
+                let mut signal = Signal::None;
+                for item in items {
+                    // Create a new scope for each iteration so the loop variable
+                    // is isolated and break/continue clean up correctly.
+                    let previous = Rc::clone(&self.env);
+                    self.env = Rc::new(RefCell::new(Env::new(Some(previous))));
+                    self.env.borrow_mut().define(var_name.clone(), item, true);
+
+                    signal = self.execute(*body.clone())?;
+                    // Restore parent scope before checking signal
+                    let parent = self.env.borrow().parent.as_ref().unwrap().clone();
+                    self.env = parent;
+
+                    match signal {
+                        Signal::Break => { signal = Signal::None; break; }
+                        Signal::Continue => { signal = Signal::None; continue; }
+                        sig @ Signal::Return(_) => return Ok(sig),
+                        Signal::None => {}
+                    }
+                }
+                Ok(signal)
+            }
             Stmt::Function { name, params, body } => {
                 let func = Value::Function(Rc::new(Function {
                     params,
@@ -1612,5 +1648,87 @@ mod tests {
         });
         assert!(result.is_err(), "Expected error for pop on non-array");
     }
+
+    // =========================================================================
+    // For-In loops
+    // =========================================================================
+
+    #[test]
+    fn eval_forin_array_elements() {
+        // Sum elements via for-in
+        let interp = run("let sum = 0; let arr = [1, 2, 3]; for (x in arr) { sum = sum + x; }");
+        assert_num(&get_var(&interp, "sum"), 6.0);
+    }
+
+    #[test]
+    fn eval_forin_empty_array() {
+        let interp = run("let count = 0; let arr = []; for (x in arr) { count = count + 1; }");
+        assert_num(&get_var(&interp, "count"), 0.0);
+    }
+
+    #[test]
+    fn eval_forin_object_keys() {
+        let interp = run("let keys = \"\"; let obj = {a: 1, b: 2}; for (k in obj) { keys = keys + k; }");
+        match get_var(&interp, "keys") {
+            Value::String(s) => {
+                // Object iteration order is not guaranteed, so check length
+                assert_eq!(s.len(), 2);
+                assert!(s.contains('a'));
+                assert!(s.contains('b'));
+            }
+            v => panic!("Expected String, got {:?}", v),
+        }
+    }
+
+    #[test]
+    fn eval_forin_string_chars() {
+        let interp = run("let result = \"\"; for (c in \"ab\") { result = result + c; }");
+        match get_var(&interp, "result") {
+            Value::String(s) => assert_eq!(s, "ab"),
+            v => panic!("Expected String, got {:?}", v),
+        }
+    }
+
+    #[test]
+    fn eval_forin_break() {
+        let interp = run("let sum = 0; for (x in [1, 2, 3, 4]) { if (x == 3) { break; } sum = sum + x; }");
+        assert_num(&get_var(&interp, "sum"), 3.0); // 1 + 2
+    }
+
+    #[test]
+    fn eval_forin_continue() {
+        let interp = run("let sum = 0; for (x in [1, 2, 3]) { if (x == 2) { continue; } sum = sum + x; }");
+        assert_num(&get_var(&interp, "sum"), 4.0); // 1 + 3
+    }
+
+    #[test]
+    fn eval_forin_loop_var_doesnt_leak() {
+        // Loop variable should not be accessible outside the loop
+        let interp = run("for (x in [1]) { let _ = x; }");
+        // x should not exist in the outer scope
+        match get_var(&interp, "x") {
+            Value::Nil => {} // Expected: x is not defined
+            v => panic!("Expected Nil (undefined), got {:?}", v),
+        }
+    }
+
+    #[test]
+    fn eval_forin_with_object_array() {
+        // Iterate over an array of objects
+        let interp = run("
+            let arr = [{v: 10}, {v: 20}];
+            let total = 0;
+            for (obj in arr) { total = total + obj.v; }
+        ");
+        assert_num(&get_var(&interp, "total"), 30.0);
+    }
+
+    #[test]
+    fn error_forin_non_iterable() {
+        let result = std::panic::catch_unwind(|| {
+            run("for (x in 42) { let _ = x; }"); // Can't iterate a number
+        });
+        assert!(result.is_err(), "Expected error for for-in on non-iterable");
+    }
 }
 
diff --git a/src/lexer/lexer.rs b/src/lexer/lexer.rs
index 71f9215..f6a4f86 100644
--- a/src/lexer/lexer.rs
+++ b/src/lexer/lexer.rs
@@ -322,6 +322,7 @@ impl Lexer {
             "else" => TokenKind::Else,
             "while" => TokenKind::While,
             "for" => TokenKind::For,
+            "in" => TokenKind::In,
             "break" => TokenKind::Break,
             "continue" => TokenKind::Continue,
             "return" => TokenKind::Return,
@@ -534,11 +535,11 @@ mod tests {
 
     #[test]
     fn keywords() {
-        let kinds = tokenize("let const fn if else while for break continue return true false nil");
+        let kinds = tokenize("let const fn if else while for in break continue return true false nil");
         assert_eq!(kinds, vec![
             TokenKind::Let, TokenKind::Const, TokenKind::Fn,
             TokenKind::If, TokenKind::Else,
-            TokenKind::While, TokenKind::For,
+            TokenKind::While, TokenKind::For, TokenKind::In,
             TokenKind::Break, TokenKind::Continue,
             TokenKind::Return,
             TokenKind::True, TokenKind::False,
diff --git a/src/lexer/token.rs b/src/lexer/token.rs
index 38f0995..0ea8c8f 100644
--- a/src/lexer/token.rs
+++ b/src/lexer/token.rs
@@ -32,6 +32,7 @@ pub enum TokenKind {
     Else,
     While,
     For,
+    In,
     Break,
     Continue,
     Return,
diff --git a/src/parser/parser.rs b/src/parser/parser.rs
index ab05d89..4fe9a62 100644
--- a/src/parser/parser.rs
+++ b/src/parser/parser.rs
@@ -134,6 +134,20 @@ impl Parser {
 
     fn for_statement(&mut self) -> Result<Stmt, RuntimeError> {
         self.consume(TokenKind::LeftParen, "Expected '(' after 'for'.")?;
+
+        // Detect for-in: for (Identifier in expr) body
+        if let TokenKind::Identifier(var_name) = &self.peek().kind {
+            let var_name = var_name.clone();
+            if matches!(self.peek_next().kind, TokenKind::In) {
+                self.advance(); // consume identifier
+                self.advance(); // consume 'in'
+                let iterable = self.expression()?;
+                self.consume(TokenKind::RightParen, "Expected ')' after for-in expression.")?;
+                let body = Box::new(self.statement()?);
+                return Ok(Stmt::ForIn { var_name, iterable, body });
+            }
+        }
+
         let initializer = if self.match_kind(&[TokenKind::Semicolon]) {
             None
         } else if self.match_kind(&[TokenKind::Let]) {
@@ -534,6 +548,10 @@ impl Parser {
         &self.tokens[self.current]
     }
 
+    fn peek_next(&self) -> &Token {
+        &self.tokens[self.current + 1]
+    }
+
     fn previous(&self) -> &Token {
         &self.tokens[self.current - 1]
     }
@@ -1257,6 +1275,48 @@ mod tests {
             }
         }
 
+        #[test]
+        fn parse_for_in_array() {
+            match parse_stmt("for (x in arr) { print(x); }") {
+                Stmt::ForIn { var_name, iterable, body } => {
+                    assert_eq!(var_name, "x");
+                    assert!(matches!(iterable, Expr::Variable(_)));
+                    assert!(matches!(*body, Stmt::Block(_)));
+                }
+                e => panic!("Expected ForIn, got {:?}", e),
+            }
+        }
+
+        #[test]
+        fn parse_for_in_with_object_literal() {
+            match parse_stmt("for (k in {a:1,b:2}) { print(k); }") {
+                Stmt::ForIn { var_name, iterable, .. } => {
+                    assert_eq!(var_name, "k");
+                    assert!(matches!(iterable, Expr::ObjectLiteral { .. }));
+                }
+                e => panic!("Expected ForIn, got {:?}", e),
+            }
+        }
+
+        #[test]
+        fn parse_for_in_with_expression() {
+            match parse_stmt("for (v in get_list()) { }") {
+                Stmt::ForIn { var_name, .. } => {
+                    assert_eq!(var_name, "v");
+                }
+                e => panic!("Expected ForIn, got {:?}", e),
+            }
+        }
+
+        #[test]
+        fn parse_for_in_distinct_from_c_style() {
+            // "for (x = 0; ...)" is C-style for, not for-in
+            match parse_stmt("for (x = 0; x < 10; x = x + 1) { }") {
+                Stmt::For { .. } => {}
+                e => panic!("Expected C-style For, got {:?}", e),
+            }
+        }
+
         #[test]
         fn parse_block() {
             match parse_stmt("{ let x = 1; x; }") {