diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,9 +1,21 @@
-# 0.0.0.1
+## 0.1.0.0 (2023-02-09)
 
+Change type of `Integer.Natural.addOne` from
+`Integer -> Integer` to `Natural -> Positive`
+
+New functions:
+
+```haskell
+Integer.Natural.length :: [a] -> Natural
+Integer.Positive.length :: NonEmpty a -> Positive
+```
+
+## 0.0.0.1 (2023-01-16)
+
 Consolidate all the test suites into one
 
 Remove `Safe` pragmas
 
-# 0.0.0.0 (2022-11-29)
+## 0.0.0.0 (2022-11-29)
 
 Initial release
diff --git a/integer-types.cabal b/integer-types.cabal
--- a/integer-types.cabal
+++ b/integer-types.cabal
@@ -1,7 +1,7 @@
 cabal-version: 3.0
 
 name: integer-types
-version: 0.0.0.1
+version: 0.1.0.0
 
 category: Numeric
 synopsis: Integer, Natural, and Positive
diff --git a/library/Integer/Natural.hs b/library/Integer/Natural.hs
--- a/library/Integer/Natural.hs
+++ b/library/Integer/Natural.hs
@@ -9,6 +9,7 @@
     {- ** Int -} toInt, fromInt,
     {- ** Word -} toWord, fromWord,
     {- * One (1) -} one, addOne, subtractOne,
+    {- * List -} length,
   )
   where
 
@@ -18,8 +19,10 @@
 import Data.Word (Word)
 import Integer.Signed (Signed (..))
 import Numeric.Natural (Natural)
+import Integer.Positive.Unsafe (Positive)
 import Prelude (Integer)
 
+import qualified Data.List as List
 import qualified Data.Ord as Ord
 import qualified Integer.Positive as Positive
 import qualified Integer.Positive.Unsafe as Positive.Unsafe
@@ -27,10 +30,10 @@
 import qualified Prelude as Bounded (Bounded (..))
 import qualified Prelude as Num (Integral (..), Num (..))
 
-toPositive :: Natural -> Maybe Positive.Unsafe.Positive
+toPositive :: Natural -> Maybe Positive
 toPositive = Positive.fromNatural
 
-fromPositive :: Positive.Unsafe.Positive -> Natural
+fromPositive :: Positive -> Natural
 fromPositive = Positive.toNatural
 
 fromInteger :: Integer -> Maybe Natural
@@ -75,10 +78,13 @@
 one :: Natural
 one = 1
 
-addOne :: Integer -> Integer
-addOne = (Num.+ 1)
+addOne :: Natural -> Positive
+addOne x = Positive.Unsafe.fromNatural (x Num.+ 1)
 
 subtractOne :: Natural -> Maybe Signed
 subtractOne x = case x of
     0 -> Nothing
     p -> Just (subtract p 1)
+
+length :: [a] -> Natural
+length = List.foldl' (\x _ -> x Num.+ 1) 0
diff --git a/library/Integer/Positive.hs b/library/Integer/Positive.hs
--- a/library/Integer/Positive.hs
+++ b/library/Integer/Positive.hs
@@ -9,18 +9,21 @@
     {- ** Int -} toInt, fromInt,
     {- ** Word -} toWord, fromWord,
     {- * One (1) -} one, addOne, subtractOne,
+    {- * List -} length,
   )
   where
 
 import Essentials
 
 import Data.Int (Int)
+import Data.List.NonEmpty (NonEmpty (..))
 import Data.Word (Word)
 import Integer.Positive.Unsafe (Positive, addOne, one, toInteger, toNatural)
 import Integer.Signed (Signed (..))
 import Numeric.Natural (Natural)
 import Prelude (Integer)
 
+import qualified Data.List as List
 import qualified Data.Ord as Ord
 import qualified Integer.Positive.Unsafe as Unsafe
 import qualified Prelude as Bounded (Bounded (..))
@@ -71,3 +74,6 @@
 fromSigned :: Signed -> Maybe Positive
 fromSigned (Plus x) = Just x
 fromSigned _        = Nothing
+
+length :: NonEmpty a -> Positive
+length (_ :| xs) = List.foldl' (\x _ -> x Num.+ 1) 1 xs
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -13,6 +13,7 @@
 import Data.Either (Either (..))
 import Data.Int (Int)
 import Data.List (take)
+import Data.List.NonEmpty (NonEmpty ((:|)))
 import Data.Word (Word)
 import Integer.Gen (GenFinite)
 import Integer.Gen (GenIntegral)
@@ -26,6 +27,8 @@
 import qualified Data.Ord as Ord
 import qualified Hedgehog
 import qualified Integer.Gen as Gen
+import qualified Integer.Natural as Natural
+import qualified Integer.Positive as Positive
 import qualified Prelude as Bounded (Bounded (..))
 import qualified Prelude as Num (fromInteger)
 import qualified Prelude as Num (toInteger)
@@ -267,6 +270,10 @@
         it "can force an error in magnitude" $ do
             x <- force (NonZero MinusSign (throw X))
             x `shouldBe` Left X
+
+    describe "length" $ do
+        it "Natural" $ Natural.length "abc" `shouldBe` 3
+        it "Positive" $ Positive.length ('a' :| "bc") `shouldBe` 3
 
 data X = X
     deriving (Eq, Show)