diff --git a/.DS_Store b/.DS_Store
new file mode 100644
index 0000000..9d85feb
Binary files /dev/null and b/.DS_Store differ
diff --git a/Cargo.toml b/Cargo.toml
index e684aea..7845c4e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,6 +17,9 @@ include = ["Cargo.toml", "src", "README.md", "LICENSE"]
 [dependencies]
 futures-util = { version = "0.3.13", optional = true }
 
+[dev-dependencies]
+tokio = { version = "1.5.0", features = ["macros", "rt", "sync"] }
+
 [features]
 async = ["futures-util"]
 no_std = []
diff --git a/src/compose.rs b/src/compose.rs
index 38b891e..be6c4fc 100644
--- a/src/compose.rs
+++ b/src/compose.rs
@@ -26,14 +26,14 @@ impl Waiter for DelayComposer {
         self.a.start();
         self.b.start();
     }
-    fn wait(&self) -> Result<(), WaiterError> {
+    fn wait(&mut self) -> Result<(), WaiterError> {
         self.a.wait()?;
         self.b.wait()?;
         Ok(())
     }
 
     #[cfg(feature = "async")]
-    fn async_wait(&self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
+    fn async_wait(&mut self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
         use futures_util::TryFutureExt;
         Box::pin(
             futures_util::future::try_join(self.a.async_wait(), self.b.async_wait()).map_ok(|_| ()),
diff --git a/src/lib.rs b/src/lib.rs
index e2adc5d..46f97b8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,7 +2,7 @@
 extern crate alloc;
 
 use alloc::boxed::Box;
-use core::cell::RefCell;
+use core::sync::atomic::{AtomicU64, Ordering};
 
 #[cfg(not(feature = "no_std"))]
 use core::time::Duration;
@@ -41,7 +41,7 @@ pub enum WaiterError {
 /// A waiter trait, that can be used for executing a delay. Waiters need to be
 /// multi-threaded and cloneable.
 /// A waiter should not be reused twice.
-pub trait Waiter: WaiterClone + Send {
+pub trait Waiter: WaiterClone + Send + Sync {
     /// Restart the wait timer.
     /// Call after starting the waiter otherwise returns an error.
     fn restart(&mut self) -> Result<(), WaiterError> {
@@ -53,12 +53,12 @@ pub trait Waiter: WaiterClone + Send {
 
     /// Called at each cycle of the waiting cycle.
     /// Call after starting the waiter otherwise returns an error.
-    fn wait(&self) -> Result<(), WaiterError>;
+    fn wait(&mut self) -> Result<(), WaiterError>;
 
     /// Async version of [wait]. By default call the blocking wait. Should be implemented
     /// to be non-blocking.
     #[cfg(feature = "async")]
-    fn async_wait(&self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
+    fn async_wait(&mut self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
         Box::pin(future::ready(self.wait()))
     }
 }
@@ -99,15 +99,15 @@ impl Waiter for Box<dyn Waiter> {
 
     /// Called at each cycle of the waiting cycle.
     /// Call after starting the waiter otherwise returns an error.
-    fn wait(&self) -> Result<(), WaiterError> {
-        self.as_ref().wait()
+    fn wait(&mut self) -> Result<(), WaiterError> {
+        self.as_mut().wait()
     }
 
     /// Async version of [wait]. By default call the blocking wait. Should be implemented
     /// to be non-blocking.
     #[cfg(feature = "async")]
-    fn async_wait(&self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
-        self.as_ref().async_wait()
+    fn async_wait(&mut self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
+        self.as_mut().async_wait()
     }
 }
 
@@ -193,12 +193,12 @@ impl Waiter for Delay {
     fn start(&mut self) {
         self.inner.start()
     }
-    fn wait(&self) -> Result<(), WaiterError> {
+    fn wait(&mut self) -> Result<(), WaiterError> {
         self.inner.wait()
     }
 
     #[cfg(feature = "async")]
-    fn async_wait(&self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
+    fn async_wait(&mut self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
         self.inner.async_wait()
     }
 }
@@ -253,15 +253,14 @@ impl DelayBuilder {
 #[derive(Clone)]
 struct InstantWaiter {}
 impl Waiter for InstantWaiter {
-    fn wait(&self) -> Result<(), WaiterError> {
+    fn wait(&mut self) -> Result<(), WaiterError> {
         Ok(())
     }
 }
 
-#[derive(Clone)]
 struct CountTimeoutWaiter {
     max_count: u64,
-    count: Option<RefCell<u64>>,
+    count: Option<AtomicU64>,
 }
 impl CountTimeoutWaiter {
     pub fn new(max_count: u64) -> Self {
@@ -271,22 +270,35 @@ impl CountTimeoutWaiter {
         }
     }
 }
+impl Clone for CountTimeoutWaiter {
+    fn clone(&self) -> Self {
+        Self {
+            max_count: self.max_count,
+            count: self
+                .count
+                .as_ref()
+                .map(|count| AtomicU64::new(count.load(Ordering::Relaxed))),
+        }
+    }
+}
 impl Waiter for CountTimeoutWaiter {
     fn restart(&mut self) -> Result<(), WaiterError> {
-        let count = self.count.as_ref().ok_or(WaiterError::NotStarted)?;
-        count.replace(0);
-        Ok(())
+        if self.count.is_none() {
+            Err(WaiterError::NotStarted)
+        } else {
+            self.count = Some(AtomicU64::new(0));
+            Ok(())
+        }
     }
 
     fn start(&mut self) {
-        self.count = Some(RefCell::new(0));
+        self.count = Some(AtomicU64::new(0));
     }
 
-    fn wait(&self) -> Result<(), WaiterError> {
-        let count = self.count.as_ref().ok_or(WaiterError::NotStarted)?;
-        let current = *count.borrow() + 1;
-        count.replace(current);
+    fn wait(&mut self) -> Result<(), WaiterError> {
+        let count = self.count.as_mut().ok_or(WaiterError::NotStarted)?;
 
+        let current = count.fetch_add(1, Ordering::Relaxed);
         if current >= self.max_count {
             Err(WaiterError::Timeout)
         } else {
@@ -315,7 +327,7 @@ impl<F> Waiter for SideEffectWaiter<F>
 where
     F: 'static + Sync + Send + Clone + Fn() -> Result<(), WaiterError>,
 {
-    fn wait(&self) -> Result<(), WaiterError> {
+    fn wait(&mut self) -> Result<(), WaiterError> {
         (self.function)()
     }
 }
diff --git a/src/tests.rs b/src/tests.rs
index fe1fadb..8322127 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -32,6 +32,7 @@ fn counter_works() {
     let mut waiter = Delay::count_timeout(3);
     waiter.start();
 
+    assert!(waiter.wait().is_ok());
     assert!(waiter.wait().is_ok());
     assert!(waiter.wait().is_ok());
     assert!(waiter.wait().is_err());
@@ -46,11 +47,13 @@ fn clone_works() {
     waiter1.start();
     assert!(waiter1.wait().is_ok());
     assert!(waiter1.wait().is_ok());
+    assert!(waiter1.wait().is_ok());
     assert!(waiter1.wait().is_err());
 
     waiter2.start();
     assert!(waiter2.wait().is_ok());
     assert!(waiter2.wait().is_ok());
+    assert!(waiter2.wait().is_ok());
     assert!(waiter2.wait().is_err());
 }
 
@@ -115,3 +118,29 @@ fn can_send_between_threads() {
 
     rx_end.recv_timeout(Duration::from_millis(100)).unwrap();
 }
+
+#[tokio::test]
+async fn works_as_async() {
+    let mut waiter = Delay::count_timeout(5);
+
+    let (tx, mut rx) = tokio::sync::mpsc::channel(5);
+    let (tx_end, mut rx_end) = tokio::sync::mpsc::channel(1);
+
+    tokio::task::spawn(async move {
+        waiter.start();
+
+        while let Some(x) = rx.recv().await.unwrap_or(None) {
+            for _i in 1..x {
+                waiter.async_wait().await.unwrap();
+            }
+        }
+
+        tx_end.send(()).await.unwrap();
+    });
+
+    tx.send(Some(4)).await.unwrap();
+    tx.send(Some(1)).await.unwrap();
+    tx.send(None).await.unwrap();
+
+    rx_end.recv().await.unwrap();
+}
diff --git a/src/throttle.rs b/src/throttle.rs
index 7037629..10af9e1 100644
--- a/src/throttle.rs
+++ b/src/throttle.rs
@@ -1,8 +1,8 @@
 #![cfg(not(feature = "no_std"))]
 use crate::{Waiter, WaiterError};
-use std::cell::RefCell;
 use std::time::Duration;
 
+use core::sync::atomic::{AtomicU64, Ordering};
 #[cfg(feature = "async")]
 use std::{future::Future, pin::Pin};
 
@@ -96,83 +96,89 @@ impl ThrottleWaiter {
     }
 }
 impl Waiter for ThrottleWaiter {
-    fn wait(&self) -> Result<(), WaiterError> {
+    fn wait(&mut self) -> Result<(), WaiterError> {
         std::thread::sleep(self.throttle);
 
         Ok(())
     }
 
     #[cfg(feature = "async")]
-    fn async_wait(&self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
+    fn async_wait(&mut self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
         Box::pin(future::ThrottleTimerFuture::new(self.throttle))
     }
 }
 
-#[derive(Clone)]
 pub struct ExponentialBackoffWaiter {
-    next: Option<RefCell<Duration>>,
-    initial: Duration,
-    multiplier: f32,
-    cap: Duration,
+    next_as_micros: Option<AtomicU64>,
+    initial_as_micros: u64,
+    multiplier: f64,
+    cap_as_micros: u64,
 }
 impl ExponentialBackoffWaiter {
     pub fn new(initial: Duration, multiplier: f32, cap: Duration) -> Self {
         ExponentialBackoffWaiter {
-            next: None,
-            initial,
-            multiplier,
-            cap,
+            next_as_micros: None,
+            initial_as_micros: initial.as_micros() as u64,
+            multiplier: multiplier as f64,
+            cap_as_micros: cap.as_micros() as u64,
+        }
+    }
+
+    fn increment(&mut self) -> Result<Duration, WaiterError> {
+        let next = self
+            .next_as_micros
+            .as_ref()
+            .ok_or(WaiterError::NotStarted)?;
+        let current = next.load(Ordering::Relaxed);
+
+        // Find the next throttle.
+        let next = u64::max(
+            (current as f64 * self.multiplier) as u64,
+            self.cap_as_micros,
+        );
+        self.next_as_micros
+            .as_mut()
+            .unwrap()
+            .store(next, Ordering::Relaxed);
+        Ok(Duration::from_micros(current))
+    }
+}
+impl Clone for ExponentialBackoffWaiter {
+    fn clone(&self) -> Self {
+        Self {
+            next_as_micros: self
+                .next_as_micros
+                .as_ref()
+                .map(|a| AtomicU64::new(a.load(Ordering::Relaxed))),
+            ..*self
         }
     }
 }
 impl Waiter for ExponentialBackoffWaiter {
     fn restart(&mut self) -> Result<(), WaiterError> {
-        let next = self.next.as_ref().ok_or(WaiterError::NotStarted)?;
-        next.replace(self.initial);
-        Ok(())
+        if self.next_as_micros.is_none() {
+            Err(WaiterError::NotStarted)
+        } else {
+            self.next_as_micros = Some(AtomicU64::new(self.initial_as_micros));
+            Ok(())
+        }
     }
 
     fn start(&mut self) {
-        self.next = Some(RefCell::new(self.initial));
+        self.next_as_micros = Some(AtomicU64::new(self.initial_as_micros));
     }
 
-    fn wait(&self) -> Result<(), WaiterError> {
-        let next = self.next.as_ref().ok_or(WaiterError::NotStarted)?;
-        let current = *next.borrow();
-        let current_nsec = current.as_nanos() as f32;
-
-        // Find the next throttle.
-        let mut next_duration = Duration::from_nanos((current_nsec * self.multiplier) as u64);
-        if next_duration > self.cap {
-            next_duration = self.cap;
-        }
-
-        next.replace(next_duration);
-
+    fn wait(&mut self) -> Result<(), WaiterError> {
+        let current = self.increment()?;
         std::thread::sleep(current);
-
         Ok(())
     }
 
     #[cfg(feature = "async")]
-    fn async_wait(&self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
-        let next = if let Some(next) = self.next.as_ref() {
-            next
-        } else {
-            return Box::pin(std::future::ready(Err(WaiterError::NotStarted)));
-        };
-
-        let current = *next.borrow();
-        let current_nsec = current.as_nanos() as f32;
-
-        // Find the next throttle.
-        let mut next_duration = Duration::from_nanos((current_nsec * self.multiplier) as u64);
-        if next_duration > self.cap {
-            next_duration = self.cap;
+    fn async_wait(&mut self) -> Pin<Box<dyn Future<Output = Result<(), WaiterError>> + Send>> {
+        match self.increment() {
+            Ok(current) => Box::pin(future::ThrottleTimerFuture::new(current)),
+            Err(e) => Box::pin(futures_util::future::err(e)),
         }
-
-        next.replace(next_duration);
-
-        Box::pin(future::ThrottleTimerFuture::new(current))
     }
 }
diff --git a/src/timeout.rs b/src/timeout.rs
index 2dff7cd..360d754 100644
--- a/src/timeout.rs
+++ b/src/timeout.rs
@@ -24,7 +24,7 @@ impl Waiter for TimeoutWaiter {
     fn start(&mut self) {
         self.start = Some(Instant::now());
     }
-    fn wait(&self) -> Result<(), WaiterError> {
+    fn wait(&mut self) -> Result<(), WaiterError> {
         let start = self.start.ok_or(WaiterError::NotStarted)?;
         if start.elapsed() > self.timeout {
             Err(WaiterError::Timeout)