diff --git a/src/async.rs b/src/async.rs
index 44b3542..77f2c5f 100644
--- a/src/async.rs
+++ b/src/async.rs
@@ -12,6 +12,7 @@
 //! Esplora by way of `reqwest` HTTP client.
 
 use std::collections::HashMap;
+use std::marker::PhantomData;
 use std::str::FromStr;
 
 use bitcoin::consensus::{deserialize, serialize, Decodable, Encodable};
@@ -26,24 +27,30 @@ use log::{debug, error, info, trace};
 
 use reqwest::{header, Client, Response};
 
+use crate::runtime::{DefaultRuntime, Runtime};
 use crate::{
     BlockStatus, BlockSummary, Builder, Error, MerkleProof, OutputStatus, Tx, TxStatus,
     BASE_BACKOFF_MILLIS, RETRYABLE_ERROR_CODES,
 };
 
 #[derive(Debug, Clone)]
-pub struct AsyncClient {
+pub struct AsyncClient<R: Runtime = DefaultRuntime> {
     /// The URL of the Esplora Server.
     url: String,
     /// The inner [`reqwest::Client`] to make HTTP requests.
     client: Client,
     /// Number of times to retry a request
     max_retries: usize,
+
+    runtime: PhantomData<R>,
 }
 
-impl AsyncClient {
+impl<R> AsyncClient<R>
+where
+    R: Runtime,
+{
     /// Build an async client from a builder
-    pub fn from_builder(builder: Builder) -> Result<Self, Error> {
+    pub fn from_builder(builder: Builder<R>) -> Result<Self, Error> {
         let mut client_builder = Client::builder();
 
         #[cfg(not(target_arch = "wasm32"))]
@@ -72,15 +79,16 @@ impl AsyncClient {
             url: builder.base_url,
             client: client_builder.build()?,
             max_retries: builder.max_retries,
+            runtime: PhantomData,
         })
     }
 
-    /// Build an async client from the base url and [`Client`]
     pub fn from_client(url: String, client: Client) -> Self {
         AsyncClient {
             url,
             client,
             max_retries: crate::DEFAULT_MAX_RETRIES,
+            runtime: PhantomData,
         }
     }
 
@@ -433,7 +441,7 @@ impl AsyncClient {
         loop {
             match self.client.get(url).send().await? {
                 resp if attempts < self.max_retries && is_status_retryable(resp.status()) => {
-                    crate::runtime::sleep(delay).await;
+                    R::sleep(delay).await;
                     attempts += 1;
                     delay *= 2;
                 }
diff --git a/src/lib.rs b/src/lib.rs
index f1f988e..b2901e6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -85,6 +85,7 @@ pub use api::*;
 pub use blocking::BlockingClient;
 #[cfg(feature = "async")]
 pub use r#async::AsyncClient;
+use runtime::{DefaultRuntime, Runtime};
 
 /// Response status codes for which the request may be retried.
 const RETRYABLE_ERROR_CODES: [u16; 3] = [
@@ -113,7 +114,7 @@ pub fn convert_fee_rate(target: usize, estimates: HashMap<u16, f64>) -> Option<f
 }
 
 #[derive(Debug, Clone)]
-pub struct Builder {
+pub struct Builder<R: Runtime = DefaultRuntime> {
     /// The URL of the Esplora server.
     pub base_url: String,
     /// Optional URL of the proxy to use to make requests to the Esplora server
@@ -135,6 +136,8 @@ pub struct Builder {
     pub headers: HashMap<String, String>,
     /// Max retries
     pub max_retries: usize,
+    /// Async runtime, trait must implement `sleep` function, default is `tokio`
+    pub runtime: R,
 }
 
 impl Builder {
@@ -146,6 +149,31 @@ impl Builder {
             timeout: None,
             headers: HashMap::new(),
             max_retries: DEFAULT_MAX_RETRIES,
+            runtime: DefaultRuntime,
+        }
+    }
+
+    /// Build a blocking client from builder
+    #[cfg(feature = "blocking")]
+    pub fn build_blocking(self) -> BlockingClient {
+        BlockingClient::from_builder(self)
+    }
+}
+
+impl<R: Runtime> Builder<R>
+where
+    R: Runtime,
+{
+    /// New with runtime
+    #[cfg(feature = "async")]
+    pub fn new_with_runtime(base_url: &str, runtime: R) -> Self {
+        Builder {
+            base_url: base_url.to_string(),
+            proxy: None,
+            timeout: None,
+            headers: HashMap::new(),
+            max_retries: DEFAULT_MAX_RETRIES,
+            runtime,
         }
     }
 
@@ -174,15 +202,9 @@ impl Builder {
         self
     }
 
-    /// Build a blocking client from builder
-    #[cfg(feature = "blocking")]
-    pub fn build_blocking(self) -> BlockingClient {
-        BlockingClient::from_builder(self)
-    }
-
     // Build an asynchronous client from builder
     #[cfg(feature = "async")]
-    pub fn build_async(self) -> Result<AsyncClient, Error> {
+    pub fn build_async(self) -> Result<AsyncClient<R>, Error> {
         AsyncClient::from_builder(self)
     }
 }
diff --git a/src/runtime.rs b/src/runtime.rs
index 64f4e40..4f21eb3 100644
--- a/src/runtime.rs
+++ b/src/runtime.rs
@@ -1,8 +1,25 @@
 #[cfg(feature = "tokio")]
-pub use tokio::time::sleep;
+use std::time::Duration;
+
+pub trait Runtime {
+    fn sleep(duration: Duration) -> impl std::future::Future<Output = ()> + Send;
+}
+
+pub struct DefaultRuntime;
+
+#[cfg(feature = "tokio")]
+impl Runtime for DefaultRuntime {
+    async fn sleep(duration: Duration) {
+        tokio::time::sleep(duration).await;
+    }
+}
 
 #[cfg(feature = "async-std")]
-pub use async_std::task::sleep;
+pub struct AsyncStd;
 
-#[cfg(not(any(feature = "tokio", feature = "async-std")))]
-compile_error!("Either 'tokio' or 'async-std' feature must be enabled");
+#[cfg(feature = "async-std")]
+impl Runtime for AsyncStd {
+    async fn sleep(duration: Duration) {
+        async_std::task::sleep(duration).await;
+    }
+}