diff --git a/Stackage/Database.hs b/Stackage/Database.hs
index 5604dff..2f25d76 100644
--- a/Stackage/Database.hs
+++ b/Stackage/Database.hs
@@ -2,6 +2,7 @@ module Stackage.Database
     ( StackageDatabase
     , GetStackageDatabase (..)
     , SnapName (..)
+    , SnapshotId ()
     , Snapshot (..)
     , newestLTS
     , newestLTSMajor
diff --git a/Stackage/Snapshot/Diff.hs b/Stackage/Snapshot/Diff.hs
new file mode 100644
index 0000000..95224fd
--- /dev/null
+++ b/Stackage/Snapshot/Diff.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE PartialTypeSignatures #-}
+module Stackage.Snapshot.Diff
+  ( getSnapshotDiff
+  , snapshotDiff
+  , SnapshotDiff
+  , PackageName
+  , Version
+  , VersionChange(..)
+  ) where
+
+import qualified Data.HashMap.Strict as HashMap
+import           Data.Align
+import           Control.Arrow
+import           ClassyPrelude
+import           Data.These
+import           Stackage.Database (SnapshotId, PackageListingInfo(..),
+                                    GetStackageDatabase, getPackages)
+type PackageName = Text
+type Version = Text
+
+type SnapshotDiff = HashMap PackageName VersionChange
+
+newtype VersionChange = VersionChange { unVersionChange :: These Version Version }
+
+changed :: VersionChange -> Bool
+changed = these (const True) (const True) (/=) . unVersionChange
+
+getSnapshotDiff :: GetStackageDatabase m => SnapshotId -> SnapshotId -> m SnapshotDiff
+getSnapshotDiff a b = snapshotDiff <$> getPackages a <*> getPackages b
+
+snapshotDiff :: [PackageListingInfo] -> [PackageListingInfo] -> SnapshotDiff
+snapshotDiff as bs = HashMap.filter changed $ alignWith VersionChange (toMap as) (toMap bs)
+  where
+    toMap = HashMap.fromList . map (pliName &&& pliVersion)
diff --git a/stack.yaml b/stack.yaml
index ecdbb12..6bf90a2 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1 +1,3 @@
 resolver: lts-3.8
+extra-deps:
+    - these-0.6.1.0
diff --git a/stackage-server.cabal b/stackage-server.cabal
index d110a26..ead55a7 100644
--- a/stackage-server.cabal
+++ b/stackage-server.cabal
@@ -31,6 +31,7 @@ library
                      Stackage.Database.Haddock
                      Stackage.Database.Types
                      Stackage.Database.Cron
+                     Stackage.Snapshot.Diff
 
                      Handler.Home
                      Handler.Snapshots
@@ -93,6 +94,7 @@ library
                   , blaze-markup                  >= 0.7         &&   < 0.8
                   , byteable                      >= 0.1         &&   < 0.2
                   , bytestring                    >= 0.10        &&   < 0.11
+                  , classy-prelude                >= 0.12        &&   < 0.13
                   , classy-prelude-yesod          >= 0.12        &&   < 0.13
                   , conduit                       >= 1.2         &&   < 1.3
                   , conduit-extra                 >= 1.1         &&   < 1.2
@@ -123,6 +125,7 @@ library
                   , template-haskell              >= 2.10        &&   < 2.11
                   , temporary-rc                  >= 1.2         &&   < 1.3
                   , text                          >= 1.2         &&   < 1.3
+                  , these                         >= 0.6         &&   < 0.7
                   , wai                           >= 3.0         &&   < 3.1
                   , wai-extra                     >= 3.0         &&   < 3.1
                   , wai-logger                    >= 2.2         &&   < 2.3