diff --git a/lib/src/prelude.ms b/lib/src/prelude.ms
index 5e33a76..a754d54 100644
--- a/lib/src/prelude.ms
+++ b/lib/src/prelude.ms
@@ -1,5 +1,6 @@
 module Core.Prelude
 
+import Core.Protos.Append
 import Core.Protos.Display
 import Core.Protos.Order
 
@@ -35,9 +36,10 @@ extern type Str impl Order, Display
 	end
 end
 
-extern type List
+extern type List impl Append
 	extern def size end
 	extern def inspect end
+	extern def append(item) end
 
 	def first
 		self[0]
diff --git a/lib/src/protos.ms b/lib/src/protos.ms
index 367d1af..74792e9 100644
--- a/lib/src/protos.ms
+++ b/lib/src/protos.ms
@@ -21,3 +21,7 @@ proto Order
 		when order(other) < 0 then other else self end
 	end
 end
+
+proto Append
+	def append(item) end
+end
diff --git a/src/rt/eval.rs b/src/rt/eval.rs
index 2200641..e4e6446 100644
--- a/src/rt/eval.rs
+++ b/src/rt/eval.rs
@@ -901,34 +901,26 @@ impl<'syms, 'descs, 'rt> Interpreter<'syms, 'descs, 'rt> {
 							}
 						}
 						BinaryOp::Append => {
-							let Val::Obj(obj) = lhs.clone() else {
+							if self
+								.rt
+								.conforms(lhs.type_id(), self.rt.behaviors.append.proto)
+							{
+								let span = chunk.get_expr_span(expr_id);
+								let member = self.rt.behaviors.append.method;
+								self.call_member(span, lhs.clone(), member, vec![(None, rhs)])
+							} else {
 								let loc = self
 									.rt
 									.pkgs
 									.desc(self.pkg_id)
 									.loc(chunk.get_expr_span(expr_id));
-								return Err(Signal::raise(
+								Err(Signal::raise(
 									Raised::Native(Error::ProtocolError(
 										ProtocolError::NotAppendable(lhs.type_id()),
 									)),
 									vec![(String::new(), loc)],
-								));
-							};
-							let Obj::List(list) = &mut *obj.borrow_mut() else {
-								let loc = self
-									.rt
-									.pkgs
-									.desc(self.pkg_id)
-									.loc(chunk.get_expr_span(expr_id));
-								return Err(Signal::raise(
-									Raised::Native(Error::ProtocolError(
-										ProtocolError::NotAppendable(obj.borrow().type_id()),
-									)),
-									vec![(String::new(), loc)],
-								));
-							};
-							list.items.push(rhs);
-							Ok(lhs)
+								))
+							}
 						}
 						BinaryOp::Add => match (lhs, rhs) {
 							(Val::Num(lhs), Val::Num(rhs)) => Ok(Val::Num(Num(lhs.0 + rhs.0))),
diff --git a/src/rt/types.rs b/src/rt/types.rs
index 634bf70..10c2970 100644
--- a/src/rt/types.rs
+++ b/src/rt/types.rs
@@ -179,7 +179,18 @@ pub const CORE_TYPES: &[NativeTypeSpec] = &[
 	NativeTypeSpec {
 		name: "List",
 		new: Some(List::new),
-		methods: &[("size", &[], List::size), ("inspect", &[], List::inspect)],
+		methods: &[
+			("size", &[], List::size),
+			("inspect", &[], List::inspect),
+			(
+				"append",
+				&[NativeParam {
+					name: "item",
+					default: None,
+				}],
+				List::append,
+			),
+		],
 		statics: &[],
 	},
 	NativeTypeSpec {
diff --git a/src/rt/val.rs b/src/rt/val.rs
index c968140..0a35d59 100644
--- a/src/rt/val.rs
+++ b/src/rt/val.rs
@@ -296,6 +296,15 @@ impl List {
 		}
 		Ok(Str::of(&print::print_list(&printed)))
 	}
+
+	pub fn append(_interp: &mut Interpreter, val: &Val, args: Vec<Val>) -> Result<Val, Raise> {
+		let Val::Obj(obj) = val else { panic!() };
+		let Obj::List(list) = &mut *obj.borrow_mut() else {
+			panic!()
+		};
+		list.items.push(args[0].clone());
+		Ok(val.clone())
+	}
 }
 
 #[derive(Debug)]
diff --git a/src/sem/protos.rs b/src/sem/protos.rs
index d409dda..7743ed8 100644
--- a/src/sem/protos.rs
+++ b/src/sem/protos.rs
@@ -62,6 +62,7 @@ pub struct Behaviors {
 	pub order: Behavior,
 	pub display: Behavior,
 	pub inspect: Behavior,
+	pub append: Behavior,
 }
 
 impl Behaviors {
@@ -72,6 +73,7 @@ impl Behaviors {
 	// Automatic: every type has this method without opting in, so the type pass
 	// seeds it long before the protocols can be resolved.
 	pub const INSPECT: (&str, &str) = ("Inspect", "inspect");
+	pub const APPEND: (&str, &str) = ("Append", "append");
 }
 
 // Resolve the protocols of 'Core.Protos' governing built-in behavior, and the
@@ -86,6 +88,7 @@ pub fn build_behaviors(syms: &mut Interner, pkgs: &Packages, stdlib: PackageId)
 		order: build_behavior(syms, pkgs, stdlib, module, Behaviors::ORDER),
 		display: build_behavior(syms, pkgs, stdlib, module, Behaviors::DISPLAY),
 		inspect: build_behavior(syms, pkgs, stdlib, module, Behaviors::INSPECT),
+		append: build_behavior(syms, pkgs, stdlib, module, Behaviors::APPEND),
 	}
 }
 
diff --git a/tests/proto.ms b/tests/proto.ms
index b5602e5..2517c71 100644
--- a/tests/proto.ms
+++ b/tests/proto.ms
@@ -210,6 +210,33 @@ $print(Task(1) >= Task(5))
 
 #---
 
+import Core.Protos.Append
+
+type Collection(items: [])
+	impl Append
+
+	def append(item)
+		items << item
+		self
+	end
+end
+
+#> Collection(items: ["Item"])
+$print(Collection() << "Item")
+#> Collection(items: ["a", "b"])
+$print(Collection() << "a" << "b")
+
+#---
+
+#! runtime error: type Task is not appendable
+
+type Task(priority)
+end
+
+Task(1) << 1
+
+#---
+
 import Core.Protos.Display
 
 type Task(priority)