diff --git a/addressbook.cabal b/addressbook.cabal
index 76cc497..a1b4f92 100644
--- a/addressbook.cabal
+++ b/addressbook.cabal
@@ -21,6 +21,7 @@ library
                      , Data.Email.Header
                      , Data.Email
                      , Control.Addressbook.Streaming
+                     , Control.Addressbook.Query
   -- other-modules:
   -- other-extensions:
   default-extensions:  OverloadedStrings
diff --git a/app/Main.hs b/app/Main.hs
index b55c6ee..f742961 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -3,18 +3,28 @@ module Main where
 
 import Options.Applicative
 
+import Data.Text
+       (Text)
+import qualified Data.Text as T
+
+import qualified Control.Addressbook.Query as Query
 import qualified Control.Addressbook.Streaming as Streaming
 
 data CmdLine
   = Stream
+  | Query Text
   deriving Show
 
 cmdline :: Parser CmdLine
-cmdline = subparser (command "stream" (info (pure Stream) (progDesc "Record a stream of filenames")))
+cmdline = subparser
+  (  command "stream" (info (pure Stream) (progDesc "Record a stream of filenames"))
+  <> command "query" (info (Query . T.pack <$> argument str (metavar "QUERY")) (progDesc "Query email addresses"))
+  )
 
 handler :: CmdLine -> IO ()
 handler = \case
   Stream -> Streaming.run
+  Query q -> Query.query q
 
 main :: IO ()
 main = execParser opts >>= handler
diff --git a/default.nix b/default.nix
index 1ab2289..4df2eca 100644
--- a/default.nix
+++ b/default.nix
@@ -1,7 +1,7 @@
 { mkDerivation, attoparsec, base, bytestring, bytestring-trie
-, conduit, conduit-extra, containers, hedgehog, hedgehog-corpus
-, HUnit, lens, mtl, optparse-applicative, stdenv, tasty
-, tasty-hedgehog, tasty-hunit, text, vector
+, conduit, conduit-extra, containers, filepath, hedgehog
+, hedgehog-corpus, HUnit, lens, mtl, optparse-applicative, stdenv
+, tasty, tasty-hedgehog, tasty-hunit, text, vector
 }:
 mkDerivation {
   pname = "addressbook";
@@ -11,9 +11,9 @@ mkDerivation {
   isExecutable = true;
   libraryHaskellDepends = [
     attoparsec base bytestring bytestring-trie conduit conduit-extra
-    containers lens mtl text vector
+    containers filepath lens mtl text vector
   ];
-  executableHaskellDepends = [ base optparse-applicative ];
+  executableHaskellDepends = [ base optparse-applicative text ];
   testHaskellDepends = [
     base bytestring conduit conduit-extra hedgehog hedgehog-corpus
     HUnit tasty tasty-hedgehog tasty-hunit text vector
diff --git a/src/Control/Addressbook/Query.hs b/src/Control/Addressbook/Query.hs
new file mode 100644
index 0000000..02ce4c3
--- /dev/null
+++ b/src/Control/Addressbook/Query.hs
@@ -0,0 +1,48 @@
+{-# LANGUAGE TypeApplications #-}
+module Control.Addressbook.Query where
+
+import Data.Text
+       (Text)
+import qualified Data.Text as T
+import qualified Data.Text.Encoding as TE
+
+import Conduit
+import qualified Data.Conduit.Binary as CB
+import qualified Data.Conduit.Combinators as C
+import qualified Data.Conduit.List as CL
+import qualified Data.Conduit.Text as CT
+
+import qualified Data.Trie as Trie
+
+import System.IO
+       (stdout)
+
+import Data.Maybe
+       (fromMaybe)
+import System.Environment
+       (lookupEnv)
+import System.FilePath
+       ((</>))
+
+import Control.Exception
+       (catch)
+
+query :: Text -> IO ()
+query prefix = do
+  datDir <- fromMaybe "./" <$> lookupEnv "HOME"
+  state <- catch @IOError (runResourceT $ runConduit $ readState datDir) (\_ -> pure Trie.empty)
+  runConduit $ outputResults state
+  where
+    readState :: FilePath -> ConduitM () Void (ResourceT IO) (Trie.Trie [Text])
+    readState dir =
+      CB.sourceFile (dir </> ".addressbook.dat")
+        .| CT.decode CT.utf8
+        .| CT.lines
+        .| C.foldMap (\s -> Trie.singleton (TE.encodeUtf8 $ T.toLower s) [s])
+    outputResults :: Trie.Trie [Text] -> ConduitM () Void IO ()
+    outputResults state =
+      CL.sourceList (Trie.elems $ Trie.submap (TE.encodeUtf8 prefix) state)
+        .| C.concat
+        .| C.map (<> "\n")
+        .| CT.encode CT.utf8
+        .| CB.sinkHandle stdout