diff --git a/README.md b/README.md
index 406a0d2..88ed955 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,16 @@
 # Flix Extras
 
 A collection of functionality that extend the official Flix library.
+
+## Modules
+
+- `Extras.Queue` — an immutable first-in, first-out queue.
+
+## Usage
+
+Add the package to the `flix.toml` of your own project:
+
+```toml
+[dependencies]
+"github:flix/extras" = "0.1.0"
+```
diff --git a/src/Extras.flix b/src/Extras.flix
new file mode 100644
index 0000000..d16fd49
--- /dev/null
+++ b/src/Extras.flix
@@ -0,0 +1,25 @@
+/*
+ * Copyright 2026 Flix Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+///
+/// The `Extras` module is a collection of submodules that extend the official
+/// Flix library, each offering a self-contained piece of functionality:
+///
+/// - `Extras.Queue` is an immutable first-in, first-out queue.
+///
+pub mod Extras {
+    /* empty */
+}
diff --git a/src/Extras/Queue.flix b/src/Extras/Queue.flix
new file mode 100644
index 0000000..71c5e02
--- /dev/null
+++ b/src/Extras/Queue.flix
@@ -0,0 +1,116 @@
+/*
+ * Copyright 2026 Flix Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+pub mod Extras.Queue {
+
+    ///
+    /// The Queue type.
+    ///
+    /// An immutable first-in, first-out (FIFO) queue.
+    ///
+    /// A queue is represented as a pair of lists: a front list, in queue order,
+    /// and a back list, in reverse queue order. Elements are dequeued from the
+    /// front and enqueued onto the back. When the front is exhausted the back is
+    /// reversed to become the new front, which gives `enqueue`, `dequeue`, and
+    /// `peek` amortized constant time.
+    ///
+    /// The representation maintains the invariant that the front list is empty
+    /// only if the queue itself is empty.
+    ///
+    /// Note - the constructor `Queue` should not be used directly.
+    ///
+    pub enum Queue[a] {
+        case Queue(List[a], List[a])
+    }
+
+    instance Eq[Queue[a]] with Eq[a] {
+        pub def eq(q1: Queue[a], q2: Queue[a]): Bool = Queue.toList(q1) == Queue.toList(q2)
+    }
+
+    instance ToString[Queue[a]] with ToString[a] {
+        pub def toString(q: Queue[a]): String = "Queue#{" + List.join(", ", Queue.toList(q)) + "}"
+    }
+
+    ///
+    /// Returns the empty queue.
+    ///
+    pub def empty(): Queue[a] = Queue.Queue(Nil, Nil)
+
+    ///
+    /// Returns `true` if and only if the queue `q` is empty.
+    ///
+    pub def isEmpty(q: Queue[a]): Bool = match q {
+        case Queue.Queue(front, _) => List.isEmpty(front)
+    }
+
+    ///
+    /// Returns the number of elements in the queue `q`.
+    ///
+    pub def size(q: Queue[a]): Int32 = match q {
+        case Queue.Queue(front, back) => List.length(front) + List.length(back)
+    }
+
+    ///
+    /// Returns the queue `q` with the element `x` added to its back.
+    ///
+    pub def enqueue(x: a, q: Queue[a]): Queue[a] = match q {
+        case Queue.Queue(front, back) => unit(front, x :: back)
+    }
+
+    ///
+    /// Returns `Some((x, rest))` where `x` is the element at the front of the
+    /// queue `q` and `rest` is `q` without `x`.
+    ///
+    /// Returns `None` if the queue `q` is empty.
+    ///
+    pub def dequeue(q: Queue[a]): Option[(a, Queue[a])] = match q {
+        case Queue.Queue(Nil, _)            => None
+        case Queue.Queue(x :: front, back)  => Some((x, unit(front, back)))
+    }
+
+    ///
+    /// Returns `Some(x)` where `x` is the element at the front of the queue `q`.
+    ///
+    /// Returns `None` if the queue `q` is empty.
+    ///
+    pub def peek(q: Queue[a]): Option[a] = match q {
+        case Queue.Queue(front, _) => List.head(front)
+    }
+
+    ///
+    /// Returns the elements of the queue `q` as a list, in queue order.
+    ///
+    pub def toList(q: Queue[a]): List[a] = match q {
+        case Queue.Queue(front, back) => List.append(front, List.reverse(back))
+    }
+
+    ///
+    /// Returns a queue holding the elements of the list `l`, where the head of
+    /// `l` is at the front of the queue.
+    ///
+    pub def fromList(l: List[a]): Queue[a] = Queue.Queue(l, Nil)
+
+    ///
+    /// Returns a queue holding the elements of `front` followed by the reverse of
+    /// `back`, restoring the invariant that the front is empty only if the queue
+    /// is empty.
+    ///
+    def unit(front: List[a], back: List[a]): Queue[a] = match front {
+        case Nil => Queue.Queue(List.reverse(back), Nil)
+        case _   => Queue.Queue(front, back)
+    }
+
+}
diff --git a/test/TestQueue.flix b/test/TestQueue.flix
new file mode 100644
index 0000000..aae0536
--- /dev/null
+++ b/test/TestQueue.flix
@@ -0,0 +1,222 @@
+mod TestQueue {
+
+    use Assert.{assertEq, assertNeq, assertTrue, assertFalse, assertNone, fail};
+    use Extras.Queue
+
+    /////////////////////////////////////////////////////////////////////////////
+    // empty                                                                   //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def empty01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertEq(expected = Nil, Queue.toList(q))
+
+    @Test
+    def empty02(): Unit \ Assert =
+        let q: Queue[String] = Queue.empty();
+        assertTrue(Queue.isEmpty(q))
+
+    @Test
+    def empty03(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertEq(expected = 0, Queue.size(q))
+
+    /////////////////////////////////////////////////////////////////////////////
+    // isEmpty                                                                 //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def isEmpty01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertTrue(Queue.isEmpty(q))
+
+    @Test
+    def isEmpty02(): Unit \ Assert =
+        assertFalse(Queue.isEmpty(Queue.enqueue(1, Queue.empty())))
+
+    @Test
+    def isEmpty03(): Unit \ Assert =
+        // A queue emptied by dequeue is empty again.
+        match Queue.dequeue(Queue.enqueue(1, Queue.empty())) {
+            case Some((_, rest)) => assertTrue(Queue.isEmpty(rest))
+            case None            => fail("expected a non-empty queue")
+        }
+
+    /////////////////////////////////////////////////////////////////////////////
+    // size                                                                    //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def size01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertEq(expected = 0, Queue.size(q))
+
+    @Test
+    def size02(): Unit \ Assert =
+        assertEq(expected = 1, Queue.size(Queue.enqueue(1, Queue.empty())))
+
+    @Test
+    def size03(): Unit \ Assert =
+        // Counts elements in both the front and the back list.
+        let q = Queue.enqueue(3, Queue.enqueue(2, Queue.fromList(1 :: Nil)));
+        assertEq(expected = 3, Queue.size(q))
+
+    /////////////////////////////////////////////////////////////////////////////
+    // enqueue                                                                 //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def enqueue01(): Unit \ Assert =
+        assertEq(expected = 1 :: Nil, Queue.toList(Queue.enqueue(1, Queue.empty())))
+
+    @Test
+    def enqueue02(): Unit \ Assert =
+        let q = Queue.enqueue(2, Queue.enqueue(1, Queue.empty()));
+        assertEq(expected = 1 :: 2 :: Nil, Queue.toList(q))
+
+    @Test
+    def enqueue03(): Unit \ Assert =
+        // Enqueueing leaves the original queue unchanged.
+        let q1 = Queue.fromList(1 :: Nil);
+        let q2 = Queue.enqueue(2, q1);
+        assertEq(expected = 1 :: Nil, Queue.toList(q1));
+        assertEq(expected = 1 :: 2 :: Nil, Queue.toList(q2))
+
+    /////////////////////////////////////////////////////////////////////////////
+    // dequeue                                                                 //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def dequeue01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertNone(Queue.dequeue(q))
+
+    @Test
+    def dequeue02(): Unit \ Assert =
+        match Queue.dequeue(Queue.fromList(1 :: 2 :: 3 :: Nil)) {
+            case Some((x, rest)) =>
+                assertEq(expected = 1, x);
+                assertEq(expected = 2 :: 3 :: Nil, Queue.toList(rest))
+            case None            => fail("expected a non-empty queue")
+        }
+
+    @Test
+    def dequeue03(): Unit \ Assert =
+        // Elements leave in the order they were enqueued, even when the front
+        // list must be refilled from the back list.
+        let q = Queue.enqueue(3, Queue.enqueue(2, Queue.enqueue(1, Queue.empty())));
+        match Queue.dequeue(q) {
+            case Some((x, rest)) =>
+                assertEq(expected = 1, x);
+                assertEq(expected = 2 :: 3 :: Nil, Queue.toList(rest))
+            case None            => fail("expected a non-empty queue")
+        }
+
+    /////////////////////////////////////////////////////////////////////////////
+    // peek                                                                    //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def peek01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertNone(Queue.peek(q))
+
+    @Test
+    def peek02(): Unit \ Assert =
+        assertEq(expected = Some(1), Queue.peek(Queue.fromList(1 :: 2 :: Nil)))
+
+    @Test
+    def peek03(): Unit \ Assert =
+        // Peeking finds the new front element after the back list has been
+        // reversed into the front.
+        let q = Queue.enqueue(3, Queue.enqueue(2, Queue.enqueue(1, Queue.empty())));
+        match Queue.dequeue(q) {
+            case Some((_, rest)) => assertEq(expected = Some(2), Queue.peek(rest))
+            case None            => fail("expected a non-empty queue")
+        }
+
+    /////////////////////////////////////////////////////////////////////////////
+    // toList                                                                  //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def toList01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertEq(expected = Nil, Queue.toList(q))
+
+    @Test
+    def toList02(): Unit \ Assert =
+        // Returns queue order across both the front and the back list.
+        let q = Queue.enqueue(3, Queue.enqueue(2, Queue.fromList(1 :: Nil)));
+        assertEq(expected = 1 :: 2 :: 3 :: Nil, Queue.toList(q))
+
+    @Test
+    def toList03(): Unit \ Assert =
+        // Returns queue order after the back list has been reversed into the front.
+        let q = Queue.enqueue(3, Queue.enqueue(2, Queue.enqueue(1, Queue.empty())));
+        match Queue.dequeue(q) {
+            case Some((_, rest)) => assertEq(expected = 2 :: 3 :: Nil, Queue.toList(rest))
+            case None            => fail("expected a non-empty queue")
+        }
+
+    /////////////////////////////////////////////////////////////////////////////
+    // fromList                                                                //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def fromList01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.fromList(Nil);
+        assertTrue(Queue.isEmpty(q))
+
+    @Test
+    def fromList02(): Unit \ Assert =
+        assertEq(expected = 1 :: 2 :: 3 :: Nil, Queue.toList(Queue.fromList(1 :: 2 :: 3 :: Nil)))
+
+    @Test
+    def fromList03(): Unit \ Assert =
+        // The head of the list is at the front of the queue.
+        assertEq(expected = Some("a"), Queue.peek(Queue.fromList("a" :: "b" :: Nil)))
+
+    /////////////////////////////////////////////////////////////////////////////
+    // Eq.eq                                                                   //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def eq01(): Unit \ Assert =
+        let q1: Queue[Int32] = Queue.empty();
+        let q2: Queue[Int32] = Queue.fromList(Nil);
+        assertEq(expected = q1, q2)
+
+    @Test
+    def eq02(): Unit \ Assert =
+        // Equality is by queue order, not by representation: these two queues hold
+        // the same elements but split them differently across front and back.
+        let q1 = Queue.fromList(1 :: 2 :: Nil);
+        let q2 = Queue.enqueue(2, Queue.enqueue(1, Queue.empty()));
+        assertEq(expected = q1, q2)
+
+    @Test
+    def eq03(): Unit \ Assert =
+        assertNeq(unexpected = Queue.fromList(1 :: 2 :: Nil), Queue.fromList(2 :: 1 :: Nil))
+
+    /////////////////////////////////////////////////////////////////////////////
+    // ToString.toString                                                       //
+    /////////////////////////////////////////////////////////////////////////////
+
+    @Test
+    def toString01(): Unit \ Assert =
+        let q: Queue[Int32] = Queue.empty();
+        assertEq(expected = "Queue#{}", ToString.toString(q))
+
+    @Test
+    def toString02(): Unit \ Assert =
+        assertEq(expected = "Queue#{1}", ToString.toString(Queue.fromList(1 :: Nil)))
+
+    @Test
+    def toString03(): Unit \ Assert =
+        // Prints in queue order across both the front and the back list.
+        let q = Queue.enqueue(3, Queue.enqueue(2, Queue.fromList(1 :: Nil)));
+        assertEq(expected = "Queue#{1, 2, 3}", ToString.toString(q))
+
+}