diff --git a/dev/Cargo.toml b/dev/Cargo.toml
index 4f725b70..262d2280 100644
--- a/dev/Cargo.toml
+++ b/dev/Cargo.toml
@@ -164,30 +164,34 @@ bin = [
   { name = "threads2_sol", path = "../solutions/20_threads/threads2.rs" },
   { name = "threads3", path = "../exercises/20_threads/threads3.rs" },
   { name = "threads3_sol", path = "../solutions/20_threads/threads3.rs" },
-  { name = "macros1", path = "../exercises/21_macros/macros1.rs" },
-  { name = "macros1_sol", path = "../solutions/21_macros/macros1.rs" },
-  { name = "macros2", path = "../exercises/21_macros/macros2.rs" },
-  { name = "macros2_sol", path = "../solutions/21_macros/macros2.rs" },
-  { name = "macros3", path = "../exercises/21_macros/macros3.rs" },
-  { name = "macros3_sol", path = "../solutions/21_macros/macros3.rs" },
-  { name = "macros4", path = "../exercises/21_macros/macros4.rs" },
-  { name = "macros4_sol", path = "../solutions/21_macros/macros4.rs" },
-  { name = "clippy1", path = "../exercises/22_clippy/clippy1.rs" },
-  { name = "clippy1_sol", path = "../solutions/22_clippy/clippy1.rs" },
-  { name = "clippy2", path = "../exercises/22_clippy/clippy2.rs" },
-  { name = "clippy2_sol", path = "../solutions/22_clippy/clippy2.rs" },
-  { name = "clippy3", path = "../exercises/22_clippy/clippy3.rs" },
-  { name = "clippy3_sol", path = "../solutions/22_clippy/clippy3.rs" },
-  { name = "using_as", path = "../exercises/23_conversions/using_as.rs" },
-  { name = "using_as_sol", path = "../solutions/23_conversions/using_as.rs" },
-  { name = "from_into", path = "../exercises/23_conversions/from_into.rs" },
-  { name = "from_into_sol", path = "../solutions/23_conversions/from_into.rs" },
-  { name = "from_str", path = "../exercises/23_conversions/from_str.rs" },
-  { name = "from_str_sol", path = "../solutions/23_conversions/from_str.rs" },
-  { name = "try_from_into", path = "../exercises/23_conversions/try_from_into.rs" },
-  { name = "try_from_into_sol", path = "../solutions/23_conversions/try_from_into.rs" },
-  { name = "as_ref_mut", path = "../exercises/23_conversions/as_ref_mut.rs" },
-  { name = "as_ref_mut_sol", path = "../solutions/23_conversions/as_ref_mut.rs" },
+  { name = "async1", path = "../exercises/21_async/async1.rs" },
+  { name = "async1_sol", path = "../solutions/21_async/async1.rs" },
+  { name = "async2", path = "../exercises/21_async/async2.rs" },
+  { name = "async2_sol", path = "../solutions/21_async/async2.rs" },
+  { name = "macros1", path = "../exercises/22_macros/macros1.rs" },
+  { name = "macros1_sol", path = "../solutions/22_macros/macros1.rs" },
+  { name = "macros2", path = "../exercises/22_macros/macros2.rs" },
+  { name = "macros2_sol", path = "../solutions/22_macros/macros2.rs" },
+  { name = "macros3", path = "../exercises/22_macros/macros3.rs" },
+  { name = "macros3_sol", path = "../solutions/22_macros/macros3.rs" },
+  { name = "macros4", path = "../exercises/22_macros/macros4.rs" },
+  { name = "macros4_sol", path = "../solutions/22_macros/macros4.rs" },
+  { name = "clippy1", path = "../exercises/23_clippy/clippy1.rs" },
+  { name = "clippy1_sol", path = "../solutions/23_clippy/clippy1.rs" },
+  { name = "clippy2", path = "../exercises/23_clippy/clippy2.rs" },
+  { name = "clippy2_sol", path = "../solutions/23_clippy/clippy2.rs" },
+  { name = "clippy3", path = "../exercises/23_clippy/clippy3.rs" },
+  { name = "clippy3_sol", path = "../solutions/23_clippy/clippy3.rs" },
+  { name = "using_as", path = "../exercises/24_conversions/using_as.rs" },
+  { name = "using_as_sol", path = "../solutions/24_conversions/using_as.rs" },
+  { name = "from_into", path = "../exercises/24_conversions/from_into.rs" },
+  { name = "from_into_sol", path = "../solutions/24_conversions/from_into.rs" },
+  { name = "from_str", path = "../exercises/24_conversions/from_str.rs" },
+  { name = "from_str_sol", path = "../solutions/24_conversions/from_str.rs" },
+  { name = "try_from_into", path = "../exercises/24_conversions/try_from_into.rs" },
+  { name = "try_from_into_sol", path = "../solutions/24_conversions/try_from_into.rs" },
+  { name = "as_ref_mut", path = "../exercises/24_conversions/as_ref_mut.rs" },
+  { name = "as_ref_mut_sol", path = "../solutions/24_conversions/as_ref_mut.rs" },
 ]
 
 [package]
@@ -196,6 +200,9 @@ edition = "2024"
 # Don't publish the exercises on crates.io!
 publish = false
 
+[dependencies]
+tokio = { version = "1.45.0", features = ["rt-multi-thread", "macros"] }
+
 [profile.release]
 panic = "abort"
 
diff --git a/exercises/21_async/README.md b/exercises/21_async/README.md
new file mode 100644
index 00000000..81bd018e
--- /dev/null
+++ b/exercises/21_async/README.md
@@ -0,0 +1,10 @@
+# Async
+
+Rust includes built-in support for asynchronous programming. In other languages, this might be known as Promises or
+Coroutines. async programming uses async functions, which are powerful, but may require some getting used to,
+especially if you haven't used something similar in another language.
+
+The [relevant book chapter][1] is essential reading. The [tokio docs][2] are also very helpful!
+
+[1]: https://doc.rust-lang.org/book/ch17-00-async-await.html
+[2]: https://tokio.rs/tokio/tutorial
diff --git a/exercises/21_async/async1.rs b/exercises/21_async/async1.rs
new file mode 100644
index 00000000..11f697ad
--- /dev/null
+++ b/exercises/21_async/async1.rs
@@ -0,0 +1,44 @@
+// Our loyal worker works hard to create a new number.
+#[derive(Default)]
+struct Worker;
+
+struct NumberContainer {
+    number: i32,
+}
+
+impl Worker {
+    async fn work(&self) -> NumberContainer {
+        // Pretend this takes a while...
+        let new_number = 32;
+        NumberContainer { number: new_number }
+    }
+}
+
+impl NumberContainer {
+    async fn extract_number(&self) -> i32 {
+        // And this too...
+        self.number
+    }
+}
+
+// TODO: Fix the function signature!
+fn run_worker() -> i32 {
+    // TODO: Make our worker create a new number and return it.
+}
+
+fn main() {
+    // Feel free to experiment here. You may need to make some adjustments
+    // to this function, though.
+}
+
+mod tests {
+    use super::*;
+
+    // Don't worry about this attribute for now.
+    // If you want to know what this does, read the hint!
+    #[tokio::test]
+    async fn test_if_it_works() {
+        let number = run_worker().await;
+        assert_eq!(number, 32);
+    }
+}
diff --git a/exercises/21_async/async2.rs b/exercises/21_async/async2.rs
new file mode 100644
index 00000000..619fff83
--- /dev/null
+++ b/exercises/21_async/async2.rs
@@ -0,0 +1,42 @@
+use tokio::task::JoinSet;
+
+// A MultiWorker can work with the power of 5 normal workers,
+// allowing us to create 5 new numbers at once!
+struct MultiWorker;
+
+impl MultiWorker {
+    async fn start_work(&self) -> JoinSet<i32> {
+        let mut set = JoinSet::new();
+
+        for i in 30..35 {
+            // TODO: `set.spawn` accepts an async function that will return the number
+            //       we want. Implement this function as a closure!
+            set.spawn(???);
+        }
+
+        set
+    }
+}
+
+async fn run_multi_worker() -> Vec<i32> {
+    let tasks = MultiWorker.start_work().await;
+
+    // TODO: We have a bunch of tasks, how do we run them to completion
+    //       to get at the i32s they create?
+}
+
+fn main() {
+    // Feel free to experiment here. You may need to make some adjustments
+    // to this function, though.
+}
+
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn test_if_it_works() {
+        let mut numbers = run_multi_worker().await;
+        numbers.sort(); // in case tasks run out-of-order
+        assert_eq!(numbers, vec![30, 31, 32, 33, 34]);
+    }
+}
diff --git a/exercises/21_macros/README.md b/exercises/22_macros/README.md
similarity index 100%
rename from exercises/21_macros/README.md
rename to exercises/22_macros/README.md
diff --git a/exercises/21_macros/macros1.rs b/exercises/22_macros/macros1.rs
similarity index 100%
rename from exercises/21_macros/macros1.rs
rename to exercises/22_macros/macros1.rs
diff --git a/exercises/21_macros/macros2.rs b/exercises/22_macros/macros2.rs
similarity index 100%
rename from exercises/21_macros/macros2.rs
rename to exercises/22_macros/macros2.rs
diff --git a/exercises/21_macros/macros3.rs b/exercises/22_macros/macros3.rs
similarity index 100%
rename from exercises/21_macros/macros3.rs
rename to exercises/22_macros/macros3.rs
diff --git a/exercises/21_macros/macros4.rs b/exercises/22_macros/macros4.rs
similarity index 100%
rename from exercises/21_macros/macros4.rs
rename to exercises/22_macros/macros4.rs
diff --git a/exercises/22_clippy/README.md b/exercises/23_clippy/README.md
similarity index 100%
rename from exercises/22_clippy/README.md
rename to exercises/23_clippy/README.md
diff --git a/exercises/22_clippy/clippy1.rs b/exercises/23_clippy/clippy1.rs
similarity index 100%
rename from exercises/22_clippy/clippy1.rs
rename to exercises/23_clippy/clippy1.rs
diff --git a/exercises/22_clippy/clippy2.rs b/exercises/23_clippy/clippy2.rs
similarity index 100%
rename from exercises/22_clippy/clippy2.rs
rename to exercises/23_clippy/clippy2.rs
diff --git a/exercises/22_clippy/clippy3.rs b/exercises/23_clippy/clippy3.rs
similarity index 100%
rename from exercises/22_clippy/clippy3.rs
rename to exercises/23_clippy/clippy3.rs
diff --git a/exercises/23_conversions/README.md b/exercises/24_conversions/README.md
similarity index 100%
rename from exercises/23_conversions/README.md
rename to exercises/24_conversions/README.md
diff --git a/exercises/23_conversions/as_ref_mut.rs b/exercises/24_conversions/as_ref_mut.rs
similarity index 100%
rename from exercises/23_conversions/as_ref_mut.rs
rename to exercises/24_conversions/as_ref_mut.rs
diff --git a/exercises/23_conversions/from_into.rs b/exercises/24_conversions/from_into.rs
similarity index 100%
rename from exercises/23_conversions/from_into.rs
rename to exercises/24_conversions/from_into.rs
diff --git a/exercises/23_conversions/from_str.rs b/exercises/24_conversions/from_str.rs
similarity index 100%
rename from exercises/23_conversions/from_str.rs
rename to exercises/24_conversions/from_str.rs
diff --git a/exercises/23_conversions/try_from_into.rs b/exercises/24_conversions/try_from_into.rs
similarity index 100%
rename from exercises/23_conversions/try_from_into.rs
rename to exercises/24_conversions/try_from_into.rs
diff --git a/exercises/23_conversions/using_as.rs b/exercises/24_conversions/using_as.rs
similarity index 100%
rename from exercises/23_conversions/using_as.rs
rename to exercises/24_conversions/using_as.rs
diff --git a/exercises/README.md b/exercises/README.md
index 237f2f1e..564fe62b 100644
--- a/exercises/README.md
+++ b/exercises/README.md
@@ -22,6 +22,7 @@
 | iterators              | §13.2-4             |
 | smart_pointers         | §15, §16.3          |
 | threads                | §16.1-3             |
+| async                  | §17                 |
 | macros                 | §19.5               |
 | clippy                 | §21.4               |
 | conversions            | n/a                 |
diff --git a/rustlings-macros/info.toml b/rustlings-macros/info.toml
index 516fd321..f24a2b0e 100644
--- a/rustlings-macros/info.toml
+++ b/rustlings-macros/info.toml
@@ -1079,11 +1079,51 @@ original sending end.
 Related section in The Book:
 https://doc.rust-lang.org/book/ch16-02-message-passing.html"""
 
+# ASYNC
+
+[[exercises]]
+name = "async1"
+dir = "21_async"
+test = true
+hint = """
+Async functions are not the same as normal functions -- they have to be marked with a
+special bit of syntax, `async fn`. These functions don't immediately return or even
+execute, you have to encourage them to do so by calling another special bit of syntax
+on them.
+
+Another thing - an async function can't be properly called in a normal function. Think of
+it as something contagious -- everything that it touches needs to be marked as such. Keeping
+that in mind, what adjustment do you need to make to the function signature?
+
+Aside:
+`#[tokio::test]` (and `#[tokio::main]`) are "magic" attributes that automatically
+set up what we call an async runtime. The Rust compiler intentionally doesn't supply
+a default implementation of this runtime. Tokio is by far the most popular
+community-developed runtime, and this macro does a lot of the heavy lifting to let
+us use it.
+"""
+
+[[exercises]]
+name = "async2"
+dir = "21_async"
+test = true
+hint = """
+Async functions can be used to run multiple things in parallel, or to more efficiently
+run things on multiple cores. Here, we use Tokio's tasks to schedule some work to run
+at the same time. We use a `JoinSet`, which is a list of tasks that lets us decide how
+to best execute them.
+
+One of the ways to execute tasks is `JoinSet::join_all`, which even gives us a neat
+Vec that we can immediately return! You can also do this sequentially, with an iterator.
+See if you can also find a way to do it that doesn't use `JoinSet`! You have access to
+most of Tokio's task-based functionality here.
+"""
+
 # MACROS
 
 [[exercises]]
 name = "macros1"
-dir = "21_macros"
+dir = "22_macros"
 test = false
 hint = """
 When you call a macro, you need to add something special compared to a regular
@@ -1091,7 +1131,7 @@ function call."""
 
 [[exercises]]
 name = "macros2"
-dir = "21_macros"
+dir = "22_macros"
 test = false
 hint = """
 Macros don't quite play by the same rules as the rest of Rust, in terms of
@@ -1102,7 +1142,7 @@ Unlike other things in Rust, the order of "where you define a macro" versus
 
 [[exercises]]
 name = "macros3"
-dir = "21_macros"
+dir = "22_macros"
 test = false
 hint = """
 In order to use a macro outside of its module, you need to do something
@@ -1110,7 +1150,7 @@ special to the module to lift the macro out into its parent."""
 
 [[exercises]]
 name = "macros4"
-dir = "21_macros"
+dir = "22_macros"
 test = false
 hint = """
 You only need to add a single character to make this compile.
@@ -1127,7 +1167,7 @@ https://veykril.github.io/tlborm/"""
 
 [[exercises]]
 name = "clippy1"
-dir = "22_clippy"
+dir = "23_clippy"
 test = false
 strict_clippy = true
 hint = """
@@ -1144,7 +1184,7 @@ appropriate replacement constant from `std::f32::consts`."""
 
 [[exercises]]
 name = "clippy2"
-dir = "22_clippy"
+dir = "23_clippy"
 test = false
 strict_clippy = true
 hint = """
@@ -1157,7 +1197,7 @@ https://doc.rust-lang.org/std/option/#iterating-over-option"""
 
 [[exercises]]
 name = "clippy3"
-dir = "22_clippy"
+dir = "23_clippy"
 test = false
 strict_clippy = true
 hint = "No hints this time!"
@@ -1166,20 +1206,20 @@ hint = "No hints this time!"
 
 [[exercises]]
 name = "using_as"
-dir = "23_conversions"
+dir = "24_conversions"
 hint = """
 Use the `as` operator to cast one of the operands in the last line of the
 `average` function into the expected return type."""
 
 [[exercises]]
 name = "from_into"
-dir = "23_conversions"
+dir = "24_conversions"
 hint = """
 Follow the steps provided right before the `From` implementation."""
 
 [[exercises]]
 name = "from_str"
-dir = "23_conversions"
+dir = "24_conversions"
 hint = """
 The implementation of `FromStr` should return an `Ok` with a `Person` object,
 or an `Err` with an error if the string is not valid.
@@ -1196,7 +1236,7 @@ https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/reen
 
 [[exercises]]
 name = "try_from_into"
-dir = "23_conversions"
+dir = "24_conversions"
 hint = """
 Is there an implementation of `TryFrom` in the standard library that can both do
 the required integer conversion and check the range of the input?
@@ -1206,6 +1246,6 @@ types?"""
 
 [[exercises]]
 name = "as_ref_mut"
-dir = "23_conversions"
+dir = "24_conversions"
 hint = """
 Add `AsRef<str>` or `AsMut<u32>` as a trait bound to the functions."""
diff --git a/solutions/21_async/async1.rs b/solutions/21_async/async1.rs
new file mode 100644
index 00000000..e5c7f5b4
--- /dev/null
+++ b/solutions/21_async/async1.rs
@@ -0,0 +1,45 @@
+// Our loyal worker works hard to create a new number.
+#[derive(Default)]
+struct Worker;
+
+struct NumberContainer {
+    number: i32,
+}
+
+impl Worker {
+    async fn work(&self) -> NumberContainer {
+        // Pretend this takes a while...
+        let new_number = 32;
+        NumberContainer { number: new_number }
+    }
+}
+
+impl NumberContainer {
+    async fn extract_number(&self) -> i32 {
+        // And this too...
+        self.number
+    }
+}
+
+async fn run_worker() -> i32 {
+    // TODO: Make our worker create a new number and return it.
+    Worker.work().await.extract_number().await
+}
+
+fn main() {
+    // Feel free to experiment here. You may need to make some adjustments
+    // to this function, though.
+}
+
+mod tests {
+    use super::*;
+
+    // Don't worry about this attribute for now.
+    // If you want to know what this does, read the hint!
+    #[tokio::test]
+    // TODO: Fix the test function signature
+    fn test_if_it_works() {
+        let number = run_worker().await;
+        assert_eq!(number, 32);
+    }
+}
diff --git a/solutions/21_async/async2.rs b/solutions/21_async/async2.rs
new file mode 100644
index 00000000..3535158a
--- /dev/null
+++ b/solutions/21_async/async2.rs
@@ -0,0 +1,43 @@
+use tokio::task::JoinSet;
+
+// A MultiWorker can work with the power of 5 normal workers,
+// allowing us to create 5 new numbers at once!
+struct MultiWorker;
+
+impl MultiWorker {
+    async fn start_work(&self) -> JoinSet<i32> {
+        let mut set = JoinSet::new();
+
+        for i in 30..35 {
+            // TODO: `set.spawn` accepts an async function that will return the number
+            //       we want. Implement this function as a closure!
+            set.spawn(async move { i });
+        }
+
+        set
+    }
+}
+
+async fn run_multi_worker() -> Vec<i32> {
+    let tasks = MultiWorker.start_work().await;
+
+    // TODO: We have a bunch of tasks, how do we run them to completion
+    //       to get at the i32s they create?
+    tasks.join_all().await
+}
+
+fn main() {
+    // Feel free to experiment here. You may need to make some adjustments
+    // to this function, though.
+}
+
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn test_if_it_works() {
+        let mut numbers = run_multi_worker().await;
+        numbers.sort(); // in case tasks run out-of-order
+        assert_eq!(numbers, vec![30, 31, 32, 33, 34]);
+    }
+}
diff --git a/solutions/21_macros/macros1.rs b/solutions/22_macros/macros1.rs
similarity index 100%
rename from solutions/21_macros/macros1.rs
rename to solutions/22_macros/macros1.rs
diff --git a/solutions/21_macros/macros2.rs b/solutions/22_macros/macros2.rs
similarity index 100%
rename from solutions/21_macros/macros2.rs
rename to solutions/22_macros/macros2.rs
diff --git a/solutions/21_macros/macros3.rs b/solutions/22_macros/macros3.rs
similarity index 100%
rename from solutions/21_macros/macros3.rs
rename to solutions/22_macros/macros3.rs
diff --git a/solutions/21_macros/macros4.rs b/solutions/22_macros/macros4.rs
similarity index 100%
rename from solutions/21_macros/macros4.rs
rename to solutions/22_macros/macros4.rs
diff --git a/solutions/22_clippy/clippy1.rs b/solutions/23_clippy/clippy1.rs
similarity index 100%
rename from solutions/22_clippy/clippy1.rs
rename to solutions/23_clippy/clippy1.rs
diff --git a/solutions/22_clippy/clippy2.rs b/solutions/23_clippy/clippy2.rs
similarity index 100%
rename from solutions/22_clippy/clippy2.rs
rename to solutions/23_clippy/clippy2.rs
diff --git a/solutions/22_clippy/clippy3.rs b/solutions/23_clippy/clippy3.rs
similarity index 100%
rename from solutions/22_clippy/clippy3.rs
rename to solutions/23_clippy/clippy3.rs
diff --git a/solutions/23_conversions/as_ref_mut.rs b/solutions/24_conversions/as_ref_mut.rs
similarity index 100%
rename from solutions/23_conversions/as_ref_mut.rs
rename to solutions/24_conversions/as_ref_mut.rs
diff --git a/solutions/23_conversions/from_into.rs b/solutions/24_conversions/from_into.rs
similarity index 100%
rename from solutions/23_conversions/from_into.rs
rename to solutions/24_conversions/from_into.rs
diff --git a/solutions/23_conversions/from_str.rs b/solutions/24_conversions/from_str.rs
similarity index 100%
rename from solutions/23_conversions/from_str.rs
rename to solutions/24_conversions/from_str.rs
diff --git a/solutions/23_conversions/try_from_into.rs b/solutions/24_conversions/try_from_into.rs
similarity index 100%
rename from solutions/23_conversions/try_from_into.rs
rename to solutions/24_conversions/try_from_into.rs
diff --git a/solutions/23_conversions/using_as.rs b/solutions/24_conversions/using_as.rs
similarity index 100%
rename from solutions/23_conversions/using_as.rs
rename to solutions/24_conversions/using_as.rs