diff --git a/Cargo.lock b/Cargo.lock
index 05b8133..8c32cbc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1090,7 +1090,7 @@ dependencies = [
 
 [[package]]
 name = "mneme"
-version = "0.4.2"
+version = "0.5.0"
 dependencies = [
  "bytes",
  "chrono",
diff --git a/Cargo.toml b/Cargo.toml
index 5a20969..5bed7e2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "mneme"
-version = "0.4.2"
+version = "0.5.0"
 authors = ["John Wilger <john@johnwilger.com>"]
 edition = "2024"
 description = "An event-sourcing library for Rust projects."
diff --git a/src/command.rs b/src/command.rs
index 99a0808..d5bc659 100644
--- a/src/command.rs
+++ b/src/command.rs
@@ -14,7 +14,7 @@ pub trait Command: Clone {
 
     fn get_state(&self) -> Self::State;
 
-    fn set_state(&mut self, state: Self::State);
+    fn set_state(&mut self, state: &Self::State);
 
     fn mark_retry(&self) -> Self
     where
@@ -36,9 +36,11 @@ pub trait Command: Clone {
 }
 
 pub trait AggregateState<E: Event>: Debug + Sized {
-    fn apply(&self, event: &E) -> Self;
+    fn apply(&mut self, event: &E) -> &Self;
 }
 
 impl<E: Event> AggregateState<E> for () {
-    fn apply(&self, _: &E) {}
+    fn apply(&mut self, _: &E) -> &Self {
+        self
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 8cb454e..bec261b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -174,7 +174,7 @@ mod tests {
         type Error = Error;
 
         fn get_state(&self) -> Self::State {}
-        fn set_state(&mut self, _: Self::State) {}
+        fn set_state(&mut self, _: &Self::State) {}
         fn event_stream_id(&self) -> EventStreamId {
             EventStreamId(self.id)
         }
@@ -334,8 +334,8 @@ mod tests {
             self.state.clone()
         }
 
-        fn set_state(&mut self, state: Self::State) {
-            self.state = state;
+        fn set_state(&mut self, state: &Self::State) {
+            self.state = state.to_owned();
         }
 
         fn event_stream_id(&self) -> EventStreamId {
@@ -378,18 +378,17 @@ mod tests {
     }
 
     impl AggregateState<TestEvent> for StatefulCommandState {
-        fn apply(&self, event: &TestEvent) -> Self {
+        fn apply(&mut self, event: &TestEvent) -> &Self {
             match event {
-                TestEvent::FooHappened { value, .. } => Self {
-                    foo: Some(*value),
-                    ..*self
-                },
-                TestEvent::BarHappened { value, .. } => Self {
-                    bar: Some(*value),
-                    ..*self
-                },
-                _ => Self { ..*self },
+                TestEvent::FooHappened { value, .. } => {
+                    self.foo = Some(*value);
+                }
+                TestEvent::BarHappened { value, .. } => {
+                    self.bar = Some(*value);
+                }
+                _ => (),
             }
+            self
         }
     }
     #[tokio::test]
@@ -476,7 +475,7 @@ mod tests {
             EventStreamId(self.id)
         }
         fn get_state(&self) -> Self::State {}
-        fn set_state(&mut self, _: Self::State) {}
+        fn set_state(&mut self, _: &Self::State) {}
     }
 
     #[tokio::test]
diff --git a/tests/test_cases.rs b/tests/test_cases.rs
index 93ee6f3..c3a662a 100644
--- a/tests/test_cases.rs
+++ b/tests/test_cases.rs
@@ -33,7 +33,7 @@ impl Command for NoopCommand {
         EventStreamId(self.id)
     }
     fn get_state(&self) -> Self::State {}
-    fn set_state(&mut self, _: Self::State) {}
+    fn set_state(&mut self, _: &Self::State) {}
 }
 
 #[derive(Debug, thiserror::Error)]
@@ -63,7 +63,7 @@ impl Command for RejectCommand {
         EventStreamId(self.id)
     }
     fn get_state(&self) -> Self::State {}
-    fn set_state(&mut self, _: Self::State) {}
+    fn set_state(&mut self, _: &Self::State) {}
 }
 
 #[derive(Clone)]
@@ -92,7 +92,7 @@ impl Command for EventProducingCommand {
         EventStreamId(self.id)
     }
     fn get_state(&self) -> Self::State {}
-    fn set_state(&mut self, _: Self::State) {}
+    fn set_state(&mut self, _: &Self::State) {}
 }
 
 #[derive(Clone, Debug)]
@@ -102,18 +102,17 @@ pub struct StatefulCommandState {
 }
 
 impl AggregateState<TestEvent> for StatefulCommandState {
-    fn apply(&self, event: &TestEvent) -> Self {
+    fn apply(&mut self, event: &TestEvent) -> &Self {
         match event {
-            TestEvent::FooHappened { value, .. } => Self {
-                foo: Some(*value),
-                ..*self
-            },
-            TestEvent::BarHappened { value, .. } => Self {
-                bar: Some(*value),
-                ..*self
-            },
-            _ => Self { ..*self },
+            TestEvent::FooHappened { value, .. } => {
+                self.foo = Some(*value);
+            }
+            TestEvent::BarHappened { value, .. } => {
+                self.bar = Some(*value);
+            }
+            _ => (),
         }
+        self
     }
 }
 
@@ -144,8 +143,8 @@ impl Command for StatefulCommand {
         self.state.clone()
     }
 
-    fn set_state(&mut self, state: Self::State) {
-        self.state = state;
+    fn set_state(&mut self, state: &Self::State) {
+        self.state = state.to_owned();
     }
 
     fn event_stream_id(&self) -> EventStreamId {