diff --git a/Cargo.toml b/Cargo.toml
index 36dea9d..b59509a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,6 +20,8 @@ serde = ["dep:serde", "dep:serde_json"]
 axum-core = "0.4"
 http = { version = "1.0", default-features = false }
 async-trait = "0.1"
+axum = "0.7"                                         # TODO: remove
+tokio = { version = "1", features = ["sync"] }       # TODO: hide behind a feature?
 
 # Optional dependencies required for the `guards` feature.
 tower = { version = "0.4", default-features = false, optional = true }
diff --git a/src/extractors.rs b/src/extractors.rs
index 81ab239..0f53604 100644
--- a/src/extractors.rs
+++ b/src/extractors.rs
@@ -137,6 +137,12 @@ where
     type Rejection = std::convert::Infallible;
 
     async fn from_request_parts(parts: &mut Parts, _: &S) -> Result<Self, Self::Rejection> {
+        use crate::vary_middleware::{HxRequestExtracted, Notifier};
+        parts
+            .extensions
+            .get_mut::<HxRequestExtracted>()
+            .map(Notifier::notify);
+
         if parts.headers.contains_key(HX_REQUEST) {
             return Ok(HxRequest(true));
         } else {
@@ -164,6 +170,12 @@ where
     type Rejection = std::convert::Infallible;
 
     async fn from_request_parts(parts: &mut Parts, _: &S) -> Result<Self, Self::Rejection> {
+        use crate::vary_middleware::{HxTargetExtracted, Notifier};
+        parts
+            .extensions
+            .get_mut::<HxTargetExtracted>()
+            .map(Notifier::notify);
+
         if let Some(target) = parts.headers.get(HX_TARGET) {
             if let Ok(target) = target.to_str() {
                 return Ok(HxTarget(Some(target.to_string())));
diff --git a/src/lib.rs b/src/lib.rs
index 439ff68..5d4dae1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -22,3 +22,6 @@ pub use guard::*;
 pub use headers::*;
 #[doc(inline)]
 pub use responders::*;
+
+pub(crate) mod vary_middleware;
+pub use vary_middleware::vary_middleware;
diff --git a/src/vary_middleware.rs b/src/vary_middleware.rs
new file mode 100644
index 0000000..57a4352
--- /dev/null
+++ b/src/vary_middleware.rs
@@ -0,0 +1,134 @@
+use crate::{
+    headers::{HX_REQUEST_STR, HX_TARGET_STR},
+    HxError,
+};
+use axum::{extract::Request, middleware::Next, response::Response};
+use axum_core::response::IntoResponse;
+use http::{
+    header::{HeaderValue, VARY},
+    Extensions,
+};
+use std::sync::Arc;
+use tokio::sync::oneshot::{self, Receiver, Sender};
+
+const MIDDLEWARE_DOUBLE_USE: &str =
+    "Configuration error: `axum_httpx::vary_middleware` is used twice";
+
+#[derive(Clone)]
+pub(crate) struct HxRequestExtracted(Option<Arc<Sender<()>>>);
+
+#[derive(Clone)]
+pub(crate) struct HxTargetExtracted(Option<Arc<Sender<()>>>);
+
+pub trait Notifier {
+    fn sender(&mut self) -> Option<Sender<()>>;
+
+    fn notify(&mut self) {
+        if let Some(sender) = self.sender().take() {
+            sender.send(()).ok();
+        }
+    }
+}
+
+impl Notifier for HxRequestExtracted {
+    fn sender(&mut self) -> Option<Sender<()>> {
+        self.0.take().and_then(Arc::into_inner)
+    }
+}
+
+impl Notifier for HxTargetExtracted {
+    fn sender(&mut self) -> Option<Sender<()>> {
+        self.0.take().and_then(Arc::into_inner)
+    }
+}
+
+impl HxRequestExtracted {
+    fn insert_into_extensions(extensions: &mut Extensions) -> Receiver<()> {
+        let (tx, rx) = oneshot::channel();
+        if extensions.insert(Self(Some(Arc::new(tx)))).is_some() {
+            panic!("{}", MIDDLEWARE_DOUBLE_USE);
+        }
+        rx
+    }
+}
+
+impl HxTargetExtracted {
+    fn insert_into_extensions(extensions: &mut Extensions) -> Receiver<()> {
+        let (tx, rx) = oneshot::channel();
+        if extensions.insert(Self(Some(Arc::new(tx)))).is_some() {
+            panic!("{}", MIDDLEWARE_DOUBLE_USE);
+        }
+        rx
+    }
+}
+
+pub async fn vary_middleware(mut request: Request, next: Next) -> Response {
+    let hx_request_rx = HxRequestExtracted::insert_into_extensions(request.extensions_mut());
+    let hx_target_rx = HxTargetExtracted::insert_into_extensions(request.extensions_mut());
+
+    let mut response = next.run(request).await;
+
+    let mut used = Vec::with_capacity(4);
+    if hx_request_rx.await.is_ok() {
+        used.push(HX_REQUEST_STR)
+    }
+    if hx_target_rx.await.is_ok() {
+        used.push(HX_TARGET_STR)
+    }
+
+    if !used.is_empty() {
+        let value = match HeaderValue::from_str(&used.join(", ")) {
+            Ok(x) => x,
+            Err(e) => return HxError::from(e).into_response(),
+        };
+        if let Err(e) = response.headers_mut().try_append(VARY, value) {
+            return HxError::from(e).into_response();
+        }
+    }
+
+    response
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{HxRequest, HxTarget};
+    use axum::{routing::get, Router};
+
+    use super::*;
+
+    fn vary_headers(resp: &axum_test::TestResponse) -> Vec<HeaderValue> {
+        resp.iter_headers_by_name("vary").cloned().collect()
+    }
+
+    #[tokio::test]
+    async fn multiple_headers() {
+        let app = Router::new()
+            .route("/no-extractors", get(|| async { () }))
+            .route("/single-extractor", get(|_: HxRequest| async { () }))
+            // Extractors can be used multiple times e.g. in middlewares
+            .route(
+                "/repeated-extractor",
+                get(|_: HxRequest, _: HxRequest| async { () }),
+            )
+            .route(
+                "/multiple-extractors",
+                get(|_: HxRequest, _: HxTarget| async { () }),
+            )
+            .layer(axum::middleware::from_fn(vary_middleware));
+        let server = axum_test::TestServer::new(app).unwrap();
+
+        assert!(vary_headers(&server.get("/no-extractors").await).is_empty());
+        assert_eq!(
+            vary_headers(&server.get("/single-extractor").await),
+            [HX_REQUEST_STR]
+        );
+        assert_eq!(
+            vary_headers(&server.get("/repeated-extractor").await),
+            [HX_REQUEST_STR]
+        );
+        assert_eq!(
+            vary_headers(&server.get("/multiple-extractors").await),
+            [format!("{HX_REQUEST_STR}, {HX_TARGET_STR}")]
+        );
+    }
+}