diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,9 @@
 # Change Log
 
+## [1.0.4.0] - 2018-07-14
+
+- Add Monad instance for boxed vectors.
+
 ## [1.0.3.1] - 2018-07-10
 
 - Loosen upper bound on `distributive`
diff --git a/src/Data/Vector/Generic/Sized.hs b/src/Data/Vector/Generic/Sized.hs
--- a/src/Data/Vector/Generic/Sized.hs
+++ b/src/Data/Vector/Generic/Sized.hs
@@ -303,6 +303,16 @@
   (*>) = seq
   (<*) = flip seq
 
+-- | Treats a @'Data.Vector.Sized.Vector' n a@ as, essentially, a @'Finite'
+-- n -> a@, and emulates the 'Monad' instance for that function.
+--
+-- @'join' :: Vector n (Vector n a) -> Vector n a@ gets the /diagonal/ from
+-- a square "matrix".
+instance KnownNat n => Monad (Vector Boxed.Vector n) where
+  return   = replicate
+  xs >>= f = imap (\i x -> f x `index` i) xs
+  (>>)     = seq
+
 -- | The 'Semigroup' instance for sized vectors does not have the same
 -- behaviour as the 'Semigroup' instance for the unsized vectors found in the
 -- 'vectors' package. This instance has @(<>) = zipWith (<>)@, but 'vectors'
diff --git a/vector-sized.cabal b/vector-sized.cabal
--- a/vector-sized.cabal
+++ b/vector-sized.cabal
@@ -1,5 +1,5 @@
 name:                vector-sized
-version:             1.0.3.1
+version:             1.0.4.0
 synopsis:            Size tagged vectors
 description:         Please see README.md
 homepage:            http://github.com/expipiplus1/vector-sized#readme