diff --git a/src/eval/array.rs b/src/eval/array.rs
index 5cd8df821..2da1a5f47 100644
--- a/src/eval/array.rs
+++ b/src/eval/array.rs
@@ -66,6 +66,11 @@ impl Array {
         Arc::make_mut(&mut self.0).push(value);
     }
 
+    /// Whether the array contains a specific value.
+    pub fn contains(&self, value: &Value) -> bool {
+        self.0.contains(value)
+    }
+
     /// Clear the array.
     pub fn clear(&mut self) {
         if Arc::strong_count(&self.0) == 1 {
diff --git a/src/eval/dict.rs b/src/eval/dict.rs
index 03871fa08..9127b2eb9 100644
--- a/src/eval/dict.rs
+++ b/src/eval/dict.rs
@@ -61,6 +61,11 @@ impl Dict {
         Arc::make_mut(&mut self.0).insert(key, value);
     }
 
+    /// Whether the dictionary contains a specific key.
+    pub fn contains_key(&self, key: &str) -> bool {
+        self.0.contains_key(key)
+    }
+
     /// Clear the dictionary.
     pub fn clear(&mut self) {
         if Arc::strong_count(&self.0) == 1 {
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 4ccf377b2..2c864036d 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -344,6 +344,8 @@ impl Eval for BinaryExpr {
             BinOp::Leq => self.apply(ctx, scp, ops::leq),
             BinOp::Gt => self.apply(ctx, scp, ops::gt),
             BinOp::Geq => self.apply(ctx, scp, ops::geq),
+            BinOp::In => self.apply(ctx, scp, ops::in_),
+            BinOp::NotIn => self.apply(ctx, scp, ops::not_in),
             BinOp::Assign => self.assign(ctx, scp, |_, b| Ok(b)),
             BinOp::AddAssign => self.assign(ctx, scp, ops::add),
             BinOp::SubAssign => self.assign(ctx, scp, ops::sub),
diff --git a/src/eval/ops.rs b/src/eval/ops.rs
index 04a13fd11..6a8f5284b 100644
--- a/src/eval/ops.rs
+++ b/src/eval/ops.rs
@@ -335,3 +335,31 @@ pub fn compare(lhs: &Value, rhs: &Value) -> Option<Ordering> {
         _ => Option::None,
     }
 }
+
+/// Test whether one value is "in" another one.
+pub fn in_(lhs: Value, rhs: Value) -> StrResult<Value> {
+    if let Some(b) = contains(&lhs, &rhs) {
+        Ok(Bool(b))
+    } else {
+        mismatch!("cannot apply 'in' to {} and {}", lhs, rhs)
+    }
+}
+
+/// Test whether one value is "not in" another one.
+pub fn not_in(lhs: Value, rhs: Value) -> StrResult<Value> {
+    if let Some(b) = contains(&lhs, &rhs) {
+        Ok(Bool(!b))
+    } else {
+        mismatch!("cannot apply 'not in' to {} and {}", lhs, rhs)
+    }
+}
+
+/// Test for containment.
+pub fn contains(lhs: &Value, rhs: &Value) -> Option<bool> {
+    Some(match (lhs, rhs) {
+        (Value::Str(a), Value::Str(b)) => b.contains(a.as_str()),
+        (Value::Str(a), Value::Dict(b)) => b.contains_key(a),
+        (a, Value::Array(b)) => b.contains(a),
+        _ => return Option::None,
+    })
+}
diff --git a/src/library/mod.rs b/src/library/mod.rs
index 505f1a1d1..087ff7eaa 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -79,7 +79,7 @@ pub fn new() -> Scope {
     std.def_fn("max", utility::max);
     std.def_fn("even", utility::even);
     std.def_fn("odd", utility::odd);
-    std.def_fn("mod", utility::modulo);
+    std.def_fn("mod", utility::mod_);
     std.def_fn("range", utility::range);
     std.def_fn("rgb", utility::rgb);
     std.def_fn("cmyk", utility::cmyk);
diff --git a/src/library/utility/math.rs b/src/library/utility/math.rs
index 9389b4b95..e48af4268 100644
--- a/src/library/utility/math.rs
+++ b/src/library/utility/math.rs
@@ -59,7 +59,7 @@ pub fn odd(_: &mut Context, args: &mut Args) -> TypResult<Value> {
 }
 
 /// The modulo of two numbers.
-pub fn modulo(_: &mut Context, args: &mut Args) -> TypResult<Value> {
+pub fn mod_(_: &mut Context, args: &mut Args) -> TypResult<Value> {
     let Spanned { v: v1, span: span1 } = args.expect("integer or float")?;
     let Spanned { v: v2, span: span2 } = args.expect("integer or float")?;
 
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index f07fefce3..5eaba8b04 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -376,9 +376,18 @@ fn expr_prec(p: &mut Parser, atomic: bool, min_prec: usize) -> ParseResult {
             with_expr(p, marker)?;
         }
 
-        let op = match p.peek().and_then(BinOp::from_token) {
-            Some(binop) => binop,
-            None => break,
+        let op = if p.eat_if(&NodeKind::Not) {
+            if p.at(&NodeKind::In) {
+                BinOp::NotIn
+            } else {
+                p.expected("keyword `in`");
+                return Err(ParseError);
+            }
+        } else {
+            match p.peek().and_then(BinOp::from_token) {
+                Some(binop) => binop,
+                None => break,
+            }
         };
 
         let mut prec = op.precedence();
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
index 928680e57..b87805908 100644
--- a/src/syntax/ast.rs
+++ b/src/syntax/ast.rs
@@ -551,9 +551,17 @@ node! {
 impl BinaryExpr {
     /// The binary operator: `+`.
     pub fn op(&self) -> BinOp {
+        let mut not = false;
         self.0
             .children()
-            .find_map(|node| BinOp::from_token(node.kind()))
+            .find_map(|node| match node.kind() {
+                NodeKind::Not => {
+                    not = true;
+                    None
+                }
+                NodeKind::In if not => Some(BinOp::NotIn),
+                _ => BinOp::from_token(node.kind()),
+            })
             .expect("binary expression is missing operator")
     }
 
@@ -601,6 +609,10 @@ pub enum BinOp {
     Geq,
     /// The assignment operator: `=`.
     Assign,
+    /// The containment operator: `in`.
+    In,
+    /// The inversed containment operator: `not in`.
+    NotIn,
     /// The add-assign operator: `+=`.
     AddAssign,
     /// The subtract-assign oeprator: `-=`.
@@ -628,6 +640,7 @@ impl BinOp {
             NodeKind::Gt => Self::Gt,
             NodeKind::GtEq => Self::Geq,
             NodeKind::Eq => Self::Assign,
+            NodeKind::In => Self::In,
             NodeKind::PlusEq => Self::AddAssign,
             NodeKind::HyphEq => Self::SubAssign,
             NodeKind::StarEq => Self::MulAssign,
@@ -649,6 +662,8 @@ impl BinOp {
             Self::Leq => 4,
             Self::Gt => 4,
             Self::Geq => 4,
+            Self::In => 4,
+            Self::NotIn => 4,
             Self::And => 3,
             Self::Or => 2,
             Self::Assign => 1,
@@ -674,6 +689,8 @@ impl BinOp {
             Self::Leq => Associativity::Left,
             Self::Gt => Associativity::Left,
             Self::Geq => Associativity::Left,
+            Self::In => Associativity::Left,
+            Self::NotIn => Associativity::Left,
             Self::Assign => Associativity::Right,
             Self::AddAssign => Associativity::Right,
             Self::SubAssign => Associativity::Right,
@@ -697,6 +714,8 @@ impl BinOp {
             Self::Leq => "<=",
             Self::Gt => ">",
             Self::Geq => ">=",
+            Self::In => "in",
+            Self::NotIn => "not in",
             Self::Assign => "=",
             Self::AddAssign => "+=",
             Self::SubAssign => "-=",
diff --git a/tests/typ/code/ops.typ b/tests/typ/code/ops.typ
index b81fc8412..899ee71c1 100644
--- a/tests/typ/code/ops.typ
+++ b/tests/typ/code/ops.typ
@@ -165,7 +165,26 @@
 { x += "thing" } #test(x, "something")
 
 ---
-// Test with operator.
+// Test `in` operator.
+#test("hi" in "worship", true)
+#test("hi" in ("we", "hi", "bye"), true)
+#test("Hey" in "abHeyCd", true)
+#test("Hey" in "abheyCd", false)
+#test(5 in range(10), true)
+#test(12 in range(10), false)
+#test("" in (), false)
+#test("key" in (key: "value"), true)
+#test("value" in (key: "value"), false)
+#test("Hey" not in "abheyCd", true)
+#test("a" not
+/* fun comment? */ in "abc", false)
+
+---
+// Error: 9 expected keyword `in`
+{"a" not}
+
+---
+// Test `with` operator.
 
 // Apply positional arguments.
 #let add(x, y) = x + y