diff --git a/block2/CHANGELOG.md b/block2/CHANGELOG.md
index e292d5278..70468145f 100644
--- a/block2/CHANGELOG.md
+++ b/block2/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased - YYYY-MM-DD
 
+### Added
+* Added `BlockMut` to allow representing cases where you know that you're not
+  modifying the block concurrently, and as such may mutate the contents.
+
 ### Fixed
 * **BREAKING**: Cleaned up `BlockArguments` trait, it is now sealed and a
   subtrait of `EncodeArguments`.
diff --git a/block2/src/block.rs b/block2/src/block.rs
index a5cb9566e..966419b4f 100644
--- a/block2/src/block.rs
+++ b/block2/src/block.rs
@@ -20,7 +20,7 @@ pub unsafe trait BlockArguments: EncodeArguments + Sized {
     #[doc(hidden)]
     unsafe fn __call_block<R: Encode>(
         invoke: unsafe extern "C" fn(),
-        block: *mut Block<Self, R>,
+        block: *mut BlockMut<Self, R>,
         args: Self,
     ) -> R;
 }
@@ -29,9 +29,9 @@ macro_rules! block_args_impl {
     ($($a:ident: $t:ident),*) => (
         unsafe impl<$($t: Encode),*> BlockArguments for ($($t,)*) {
             #[inline]
-            unsafe fn __call_block<R: Encode>(invoke: unsafe extern "C" fn(), block: *mut Block<Self, R>, ($($a,)*): Self) -> R {
+            unsafe fn __call_block<R: Encode>(invoke: unsafe extern "C" fn(), block: *mut BlockMut<Self, R>, ($($a,)*): Self) -> R {
                 // Very similar to `MessageArguments::__invoke`
-                let invoke: unsafe extern "C" fn(*mut Block<Self, R> $(, $t)*) -> R = unsafe {
+                let invoke: unsafe extern "C" fn(*mut BlockMut<Self, R> $(, $t)*) -> R = unsafe {
                     mem::transmute(invoke)
                 };
 
@@ -80,39 +80,80 @@ block_args_impl!(
     l: L
 );
 
-/// An Objective-C block that takes arguments of `A` when called and
-/// returns a value of `R`.
+/// A block that can be called repeatedly and may mutate state.
+///
+/// This is the Objective-C equivalent of [`dyn FnMut`][FnMut].
+///
+/// Takes arguments `A` and returns `R`.
 #[repr(C)]
-pub struct Block<A, R> {
+pub struct BlockMut<A, R> {
     _inner: [u8; 0],
-    // We effectively store `Block_layout` + a bit more, but `Block` has to
-    // remain an empty type otherwise the compiler thinks we only have
-    // provenance over `Block_layout`.
+    /// We effectively store `Block_layout` + a bit more, but `Block` has to
+    /// remain an empty type otherwise the compiler thinks we only have
+    /// provenance over `Block_layout`.
     _layout: PhantomData<ffi::Block_layout>,
-    // To get correct variance on args and return types
-    _p: PhantomData<fn(A) -> R>,
+    /// To get correct variance on args and return types
+    _p: PhantomData<dyn FnMut(A) -> R>,
 }
 
-unsafe impl<A: BlockArguments, R: Encode> RefEncode for Block<A, R> {
+unsafe impl<A: BlockArguments, R: Encode> RefEncode for BlockMut<A, R> {
     const ENCODING_REF: Encoding<'static> = Encoding::Block;
 }
 
-impl<A: BlockArguments, R: Encode> Block<A, R> {
-    /// Call self with the given arguments.
-    ///
-    /// # Safety
-    ///
-    /// This invokes foreign code that the caller must verify doesn't violate
-    /// any of Rust's safety rules.
-    ///
-    /// For example, if this block is shared with multiple references, the
-    /// caller must ensure that calling it will not cause a data race.
-    pub unsafe fn call(&self, args: A) -> R {
+impl<A: BlockArguments, R: Encode> BlockMut<A, R> {
+    fn get_invoke(&self) -> unsafe extern "C" fn() {
         let ptr: *const Self = self;
-        let layout = unsafe { ptr.cast::<ffi::Block_layout>().as_ref().unwrap_unchecked() };
+        let ptr: *const ffi::Block_layout = ptr.cast();
+        // SAFETY: `BlockMut` is `Block_layout` + extern type
+        let layout = unsafe { ptr.as_ref().unwrap_unchecked() };
         // TODO: Is `invoke` actually ever null?
-        let invoke = layout.invoke.unwrap();
+        layout.invoke.unwrap()
+    }
+
+    /// Performs the call operation.
+    pub fn call_mut(&mut self, args: A) -> R {
+        let invoke = self.get_invoke();
+        let ptr: *mut Self = self;
+
+        unsafe { A::__call_block(invoke, ptr.cast(), args) }
+    }
+}
+
+/// A block that can be called repeatedly without mutating state.
+///
+/// This is the Objective-C equivalent of [`dyn Fn`][Fn].
+///
+/// Takes arguments `A` and returns `R`.
+#[repr(C)]
+pub struct Block<A, R> {
+    /// Same as BlockMut, with the added ability that this may be called
+    /// immutably.
+    inner: BlockMut<A, R>,
+}
+
+unsafe impl<A: BlockArguments, R: Encode> RefEncode for Block<A, R> {
+    const ENCODING_REF: Encoding<'static> = Encoding::Block;
+}
+
+impl<A: BlockArguments, R: Encode> Block<A, R> {
+    /// Performs the call operation.
+    pub fn call(&self, args: A) -> R {
+        let invoke = self.inner.get_invoke();
+        let ptr: *const BlockMut<A, R> = &self.inner;
+
+        // SAFETY: `Block` is safe to call from an immutable reference
+        unsafe { A::__call_block(invoke, ptr as *mut BlockMut<A, R>, args) }
+    }
+}
+
+impl<A, R> AsRef<BlockMut<A, R>> for Block<A, R> {
+    fn as_ref(&self) -> &BlockMut<A, R> {
+        &self.inner
+    }
+}
 
-        unsafe { A::__call_block(invoke, ptr as *mut Self, args) }
+impl<A, R> AsMut<BlockMut<A, R>> for Block<A, R> {
+    fn as_mut(&mut self) -> &mut BlockMut<A, R> {
+        &mut self.inner
     }
 }
diff --git a/block2/src/debug.rs b/block2/src/debug.rs
index 2878ba3d6..c616954d6 100644
--- a/block2/src/debug.rs
+++ b/block2/src/debug.rs
@@ -4,7 +4,7 @@ use core::fmt::{Debug, DebugStruct, Error, Formatter};
 use core::ptr;
 use std::ffi::CStr;
 
-use crate::{ffi, Block, ConcreteBlock, GlobalBlock, RcBlock};
+use crate::{ffi, Block, BlockMut, ConcreteBlock, GlobalBlock, RcBlock};
 
 #[derive(Clone, Copy, PartialEq, Eq)]
 struct Isa(*const ffi::Class);
@@ -52,6 +52,16 @@ fn debug_block_layout(layout: &ffi::Block_layout, f: &mut DebugStruct<'_, '_>) {
     );
 }
 
+impl<A, R> Debug for BlockMut<A, R> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
+        let mut f = f.debug_struct("BlockMut");
+        let ptr: *const Self = self;
+        let layout = unsafe { ptr.cast::<ffi::Block_layout>().as_ref().unwrap() };
+        debug_block_layout(layout, &mut f);
+        f.finish_non_exhaustive()
+    }
+}
+
 impl<A, R> Debug for Block<A, R> {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
         let mut f = f.debug_struct("Block");
diff --git a/block2/src/global.rs b/block2/src/global.rs
index baca564e6..1f9b8f85b 100644
--- a/block2/src/global.rs
+++ b/block2/src/global.rs
@@ -29,7 +29,7 @@ const GLOBAL_DESCRIPTOR: ffi::Block_descriptor_header = ffi::Block_descriptor_he
 #[repr(C)]
 pub struct GlobalBlock<A, R = ()> {
     pub(crate) layout: ffi::Block_layout,
-    p: PhantomData<(A, R)>,
+    p: PhantomData<Block<A, R>>,
 }
 
 unsafe impl<A, R> Sync for GlobalBlock<A, R>
@@ -85,7 +85,10 @@ where
     fn deref(&self) -> &Self::Target {
         let ptr: *const Self = self;
         let ptr: *const Block<A, R> = ptr.cast();
-        // TODO: SAFETY
+        // SAFETY: This has the same layout as `Block`
+        //
+        // A global block does not hold any data, so it is safe to call
+        // immutably.
         unsafe { ptr.as_ref().unwrap_unchecked() }
     }
 }
@@ -105,7 +108,7 @@ where
 ///         42
 ///     };
 /// }
-/// assert_eq!(unsafe { MY_BLOCK.call(()) }, 42);
+/// assert_eq!(MY_BLOCK.call(()), 42);
 /// ```
 ///
 /// ```
@@ -115,7 +118,7 @@ where
 ///         x + y
 ///     };
 /// }
-/// assert_eq!(unsafe { ADDER_BLOCK.call((5, 7)) }, 12);
+/// assert_eq!(ADDER_BLOCK.call((5, 7)), 12);
 /// ```
 ///
 /// ```
@@ -126,7 +129,7 @@ where
 ///     };
 /// }
 /// let mut x = 5;
-/// unsafe { MUTATING_BLOCK.call((&mut x,)) };
+/// MUTATING_BLOCK.call((&mut x,));
 /// assert_eq!(x, 47);
 /// ```
 ///
@@ -206,7 +209,7 @@ mod tests {
 
     #[test]
     fn test_noop_block() {
-        unsafe { NOOP_BLOCK.call(()) };
+        NOOP_BLOCK.call(());
     }
 
     #[test]
@@ -214,7 +217,7 @@ mod tests {
         global_block!(static MY_BLOCK = || -> i32 {
             42
         });
-        assert_eq!(unsafe { MY_BLOCK.call(()) }, 42);
+        assert_eq!(MY_BLOCK.call(()), 42);
     }
 
     #[cfg(feature = "apple")]
diff --git a/block2/src/lib.rs b/block2/src/lib.rs
index f4ef3b3dd..9ad84ce44 100644
--- a/block2/src/lib.rs
+++ b/block2/src/lib.rs
@@ -103,7 +103,7 @@ mod debug;
 mod global;
 mod rc_block;
 
-pub use block::{Block, BlockArguments};
+pub use block::{Block, BlockArguments, BlockMut};
 pub use concrete_block::{ConcreteBlock, IntoConcreteBlock};
 pub use global::GlobalBlock;
 pub use rc_block::RcBlock;