diff --git a/src/codec.rs b/src/codec.rs
index f8fb40d..df3a31c 100644
--- a/src/codec.rs
+++ b/src/codec.rs
@@ -83,9 +83,8 @@ where
     for<'a> R: Region<ReadItem<'a> = &'a [u8]> + 'a,
     for<'a> &'a [u8]: CopyOnto<R>,
 {
-    type ReadItem<'a> = &'a [u8]
-    where
-        Self: 'a;
+    type ReadItem<'a> = &'a [u8] where Self: 'a;
+    type ReadItemMut<'a> = Self::ReadItem<'a> where Self: 'a;
 
     type Index = R::Index;
 
@@ -106,6 +105,10 @@ where
         self.codec.decode(self.inner.index(index))
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        self.index(index)
+    }
+
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
         Self: 'a,
diff --git a/src/impls/columns.rs b/src/impls/columns.rs
index 618939d..1919721 100644
--- a/src/impls/columns.rs
+++ b/src/impls/columns.rs
@@ -40,6 +40,7 @@ where
     Idx: Index,
 {
     type ReadItem<'a> = ReadColumns<'a, R, Idx> where Self: 'a;
+    type ReadItemMut<'a> = ReadColumnsMut<'a, R, Idx> where Self: 'a;
     type Index = usize;
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -68,6 +69,13 @@ where
         }
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        ReadColumnsMut {
+            columns: &mut self.inner,
+            index: self.indices.index(index),
+        }
+    }
+
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
         Self: 'a,
@@ -104,6 +112,72 @@ where
     }
 }
 
+/// Read the values of a row.
+pub struct ReadColumnsMut<'a, R, Idx>
+where
+    R: Region<Index = Idx>,
+    Idx: Index,
+{
+    /// Storage for columns.
+    columns: &'a mut [R],
+    /// Indices to retrieve values from columns.
+    index: &'a [Idx],
+}
+
+impl<'a, R, Idx> ReadColumnsMut<'a, R, Idx>
+where
+    R: Region<Index = Idx>,
+    Idx: Index,
+{
+    /// Iterate the individual values of a row.
+    pub fn iter(&'a mut self) -> impl Iterator<Item = R::ReadItemMut<'a>> {
+        self.index
+            .iter()
+            .zip(self.columns.iter_mut())
+            .map(|(idx, r)| r.index_mut(*idx))
+    }
+
+    /// Get the element at `offset`.
+    pub fn get(&'a mut self, offset: usize) -> R::ReadItemMut<'a> {
+        self.columns[offset].index_mut(self.index[offset])
+    }
+
+    /// Returns the length of this row.
+    pub fn len(&self) -> usize {
+        self.index.len()
+    }
+
+    /// Returns `true` if this row is empty.
+    pub fn is_empty(&self) -> bool {
+        self.index.is_empty()
+    }
+}
+
+impl<'a, R, Idx> CopyOnto<ColumnsRegion<R, Idx>> for ReadColumnsMut<'a, R, Idx>
+where
+    R: Region<Index = Idx>,
+    Idx: Index,
+{
+    fn copy_onto(
+        self,
+        target: &mut ColumnsRegion<R, Idx>,
+    ) -> <ColumnsRegion<R, Idx> as Region>::Index {
+        // Ensure all required regions exist.
+        while target.inner.len() < self.len() {
+            target.inner.push(R::default());
+        }
+
+        let iter = self
+            .index
+            .iter()
+            .zip(self.columns.iter_mut())
+            .map(|(idx, r)| r.index_mut(*idx))
+            .zip(&mut target.inner)
+            .map(|(value, region)| value.copy_onto(region));
+        CopyIter(iter).copy_onto(&mut target.indices)
+    }
+}
+
 /// Read the values of a row.
 #[derive(Copy)]
 pub struct ReadColumns<'a, R, Idx>
diff --git a/src/impls/deduplicate.rs b/src/impls/deduplicate.rs
index 2f4cda2..055c94b 100644
--- a/src/impls/deduplicate.rs
+++ b/src/impls/deduplicate.rs
@@ -26,6 +26,11 @@ where
     for<'a, 'b> R::ReadItem<'a>: PartialEq<R::ReadItem<'b>>,
 {
     type ReadItem<'a> = R::ReadItem<'a> where Self: 'a;
+
+    // Important: We do not permit mutable access to our contents because multiple
+    // indices can reference the same data.
+    type ReadItemMut<'a> = R::ReadItem<'a> where Self: 'a;
+
     type Index = R::Index;
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -42,6 +47,10 @@ where
         self.inner.index(index)
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        self.index(index)
+    }
+
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
         Self: 'a,
@@ -109,9 +118,9 @@ impl<R: Region<Index = (usize, usize)>, O: OffsetContainer<usize>> Default
 impl<R: Region<Index = (usize, usize)>, O: OffsetContainer<usize>> Region
     for ConsecutiveOffsetPairs<R, O>
 {
-    type ReadItem<'a> = R::ReadItem<'a>
-    where
-        Self: 'a;
+    type ReadItem<'a> = R::ReadItem<'a> where Self: 'a;
+
+    type ReadItemMut<'a> = R::ReadItemMut<'a> where Self: 'a;
 
     type Index = usize;
 
@@ -133,6 +142,11 @@ impl<R: Region<Index = (usize, usize)>, O: OffsetContainer<usize>> Region
             .index((self.offsets.index(index), self.offsets.index(index + 1)))
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        self.inner
+            .index_mut((self.offsets.index(index), self.offsets.index(index + 1)))
+    }
+
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
         Self: 'a,
diff --git a/src/impls/mirror.rs b/src/impls/mirror.rs
index a4f8520..f488cd7 100644
--- a/src/impls/mirror.rs
+++ b/src/impls/mirror.rs
@@ -42,6 +42,7 @@ impl<T> Debug for MirrorRegion<T> {
 
 impl<T: Index + CopyOnto<Self>> Region for MirrorRegion<T> {
     type ReadItem<'a> = T where T: 'a;
+    type ReadItemMut<'a> = T where Self: 'a;
     type Index = T;
 
     fn merge_regions<'a>(_regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -56,6 +57,10 @@ impl<T: Index + CopyOnto<Self>> Region for MirrorRegion<T> {
         index
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        index
+    }
+
     #[inline(always)]
     fn reserve_regions<'a, I>(&mut self, _regions: I)
     where
diff --git a/src/impls/option.rs b/src/impls/option.rs
index ae476c2..e38c2d8 100644
--- a/src/impls/option.rs
+++ b/src/impls/option.rs
@@ -33,6 +33,7 @@ pub struct OptionRegion<R> {
 
 impl<R: Region> Region for OptionRegion<R> {
     type ReadItem<'a> = Option<R::ReadItem<'a>> where Self: 'a;
+    type ReadItemMut<'a> = Option<R::ReadItemMut<'a>> where Self: 'a;
     type Index = Option<R::Index>;
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -49,6 +50,10 @@ impl<R: Region> Region for OptionRegion<R> {
         index.map(|t| self.inner.index(t))
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        index.map(|t| self.inner.index_mut(t))
+    }
+
     #[inline]
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
diff --git a/src/impls/result.rs b/src/impls/result.rs
index 79bb2b9..c20f70c 100644
--- a/src/impls/result.rs
+++ b/src/impls/result.rs
@@ -38,6 +38,7 @@ where
     E: Region,
 {
     type ReadItem<'a> = Result<T::ReadItem<'a>, E::ReadItem<'a>> where Self: 'a;
+    type ReadItemMut<'a> = Result<T::ReadItemMut<'a>, E::ReadItemMut<'a>> where Self: 'a;
     type Index = Result<T::Index, E::Index>;
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -58,6 +59,13 @@ where
         }
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        match index {
+            Ok(index) => Ok(self.oks.index_mut(index)),
+            Err(index) => Err(self.errs.index_mut(index)),
+        }
+    }
+
     #[inline]
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
diff --git a/src/impls/slice.rs b/src/impls/slice.rs
index 0545dc6..e1a32b2 100644
--- a/src/impls/slice.rs
+++ b/src/impls/slice.rs
@@ -61,6 +61,7 @@ pub struct SliceRegion<C: Region, O: OffsetContainer<C::Index> = Vec<<C as Regio
 
 impl<C: Region, O: OffsetContainer<C::Index>> Region for SliceRegion<C, O> {
     type ReadItem<'a> = ReadSlice<'a, C, O> where Self: 'a;
+    type ReadItemMut<'a> = ReadSlice<'a, C, O> where Self: 'a;
     type Index = (usize, usize);
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -82,6 +83,10 @@ impl<C: Region, O: OffsetContainer<C::Index>> Region for SliceRegion<C, O> {
         }
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        self.index(index)
+    }
+
     #[inline]
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
diff --git a/src/impls/slice_copy.rs b/src/impls/slice_copy.rs
index 9309c53..04bad43 100644
--- a/src/impls/slice_copy.rs
+++ b/src/impls/slice_copy.rs
@@ -30,6 +30,7 @@ pub struct CopyRegion<T: Copy> {
 
 impl<T: Copy> Region for CopyRegion<T> {
     type ReadItem<'a> = &'a [T] where Self: 'a;
+    type ReadItemMut<'a> = &'a mut [T] where Self: 'a;
     type Index = (usize, usize);
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -46,6 +47,10 @@ impl<T: Copy> Region for CopyRegion<T> {
         &self.slices[start..end]
     }
 
+    fn index_mut(&mut self, (start, end): Self::Index) -> Self::ReadItemMut<'_> {
+        &mut self.slices[start..end]
+    }
+
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
         Self: 'a,
@@ -89,6 +94,25 @@ impl<T: Copy> ReserveItems<CopyRegion<T>> for &[T] {
     }
 }
 
+impl<T> CopyOnto<CopyRegion<T>> for &mut [T]
+where
+    T: Copy,
+{
+    #[inline]
+    fn copy_onto(self, target: &mut CopyRegion<T>) -> <CopyRegion<T> as Region>::Index {
+        (&*self).copy_onto(target)
+    }
+}
+
+impl<T: Copy> ReserveItems<CopyRegion<T>> for &mut [T] {
+    fn reserve_items<I>(target: &mut CopyRegion<T>, items: I)
+    where
+        I: Iterator<Item = Self> + Clone,
+    {
+        ReserveItems::reserve_items(target, items.map(|r| &*r));
+    }
+}
+
 impl<T> CopyOnto<CopyRegion<T>> for &Vec<T>
 where
     T: Copy,
diff --git a/src/impls/string.rs b/src/impls/string.rs
index 693ae00..2702433 100644
--- a/src/impls/string.rs
+++ b/src/impls/string.rs
@@ -32,6 +32,7 @@ pub struct StringRegion {
 
 impl Region for StringRegion {
     type ReadItem<'a> = &'a str where Self: 'a ;
+    type ReadItemMut<'a> = &'a mut str where Self: 'a;
     type Index = <CopyRegion<u8> as Region>::Index;
 
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -48,6 +49,10 @@ impl Region for StringRegion {
         unsafe { std::str::from_utf8_unchecked(self.inner.index(index)) }
     }
 
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+        unsafe { std::str::from_utf8_unchecked_mut(self.inner.index_mut(index)) }
+    }
+
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
         Self: 'a,
@@ -100,6 +105,13 @@ impl CopyOnto<StringRegion> for &str {
     }
 }
 
+impl CopyOnto<StringRegion> for &mut str {
+    #[inline]
+    fn copy_onto(self, target: &mut StringRegion) -> <StringRegion as Region>::Index {
+        <&str as CopyOnto<StringRegion>>::copy_onto(self, target)
+    }
+}
+
 impl CopyOnto<StringRegion> for &&str {
     #[inline]
     fn copy_onto(self, target: &mut StringRegion) -> <StringRegion as Region>::Index {
diff --git a/src/impls/tuple.rs b/src/impls/tuple.rs
index c143a96..cdadc24 100644
--- a/src/impls/tuple.rs
+++ b/src/impls/tuple.rs
@@ -28,6 +28,7 @@ macro_rules! tuple_flatcontainer {
                $(<$name as Region>::Index: crate::Index),*
             {
                 type ReadItem<'a> = ($($name::ReadItem<'a>,)*) where Self: 'a;
+                type ReadItemMut<'a> = ($($name::ReadItemMut<'a>,)*) where Self: 'a;
 
                 type Index = ($($name::Index,)*);
 
@@ -46,6 +47,13 @@ macro_rules! tuple_flatcontainer {
                     )
                 }
 
+                #[inline] fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+                    let ($($name,)*) = index;
+                    (
+                        $(self.[<container $name>].index_mut($name),)*
+                    )
+                }
+
                 #[inline(always)]
                 fn reserve_regions<'a, It>(&mut self, regions: It)
                 where
diff --git a/src/lib.rs b/src/lib.rs
index 387cb0b..bfc84d7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -71,6 +71,11 @@ pub trait Region: Default {
     where
         Self: 'a;
 
+    /// The type of the data that one gets out of the container.
+    type ReadItemMut<'a>: CopyOnto<Self>
+    where
+        Self: 'a;
+
     /// The type to index into the container. Should be treated
     /// as an opaque type, even if known.
     type Index: Index;
@@ -84,6 +89,10 @@ pub trait Region: Default {
     /// pushing data into the container.
     fn index(&self, index: Self::Index) -> Self::ReadItem<'_>;
 
+    /// Index into the container. The index must be obtained by
+    /// pushing data into the container.
+    fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_>;
+
     /// Ensure that the region can absorb the items of `regions` without reallocation
     fn reserve_regions<'a, I>(&mut self, regions: I)
     where
@@ -381,6 +390,7 @@ mod tests {
 
     impl Region for PersonRegion {
         type ReadItem<'a> = PersonRef<'a> where Self: 'a;
+        type ReadItemMut<'a>  = PersonRef<'a>where Self: 'a;
         type Index = (
             <<String as Containerized>::Region as Region>::Index,
             <<u16 as Containerized>::Region as Region>::Index,
@@ -412,6 +422,10 @@ mod tests {
             }
         }
 
+        fn index_mut(&mut self, index: Self::Index) -> Self::ReadItemMut<'_> {
+            self.index(index)
+        }
+
         fn reserve_regions<'a, I>(&mut self, regions: I)
         where
             Self: 'a,