diff --git a/Web/Herringbone/BuildAsset.hs b/Web/Herringbone/BuildAsset.hs
--- a/Web/Herringbone/BuildAsset.hs
+++ b/Web/Herringbone/BuildAsset.hs
@@ -2,6 +2,7 @@
 -- if necessary, and copy to destination directory).
 module Web.Herringbone.BuildAsset where
 
+import Control.Monad.Reader
 import Data.Time
 import Filesystem.Path.CurrentOS (FilePath, (</>))
 import qualified Filesystem as F
@@ -24,7 +25,7 @@
     compileNeeded <- shouldCompile sourceModifiedTime destPath
 
     result <- if compileNeeded
-                then compileAsset sourcePath destPath pps
+                then compileAsset hb logPath sourcePath destPath pps
                 else return $ Right ()
 
     either (return . Left)
@@ -50,14 +51,24 @@
 
 -- | Compile the given asset by invoking any preprocessors on the source path,
 -- and copying the result to the destination path.
-compileAsset :: FilePath -- ^ Source path
+compileAsset :: Herringbone
+             -> LogicalPath
+             -> FilePath -- ^ Source path
              -> FilePath -- ^ Dest path
              -> [PP]     -- ^ List of preprocessors to apply
              -> IO (Either CompileError ())
-compileAsset sourcePath destPath pps = do
+compileAsset hb logPath sourcePath destPath pps = do
     sourceData <- F.readFile sourcePath
 
-    result <- chainEither (map ppAction pps) sourceData
+    let computation = chainEither (map ppAction pps) sourceData
+    let readerData = PPReader
+            { ppReaderHb          = hb
+            , ppReaderLogicalPath = logPath
+            , ppReaderSourcePath  = sourcePath
+            , ppReaderPPs         = pps
+            }
+    result <- runPPM computation readerData
+
     either (return . Left)
            (\resultData -> do F.writeFile destPath resultData
                               return (Right ()))
diff --git a/Web/Herringbone/Types.hs b/Web/Herringbone/Types.hs
--- a/Web/Herringbone/Types.hs
+++ b/Web/Herringbone/Types.hs
@@ -1,5 +1,6 @@
 module Web.Herringbone.Types where
 
+import Control.Monad.Reader
 import Data.Char
 import Data.Time.Clock
 import Data.Time.Format
@@ -29,6 +30,27 @@
                 | AmbiguousSources [FilePath]
                 deriving (Show, Eq)
 
+-- | Data which is given to preprocessors on the off-chance that they need it
+-- (eg, Fay)
+data PPReader = PPReader
+    { ppReaderHb          :: Herringbone 
+    -- ^ The Herringbone which was used to build the asset
+    , ppReaderLogicalPath :: LogicalPath
+    -- ^ The Logical path of the requested asset.
+    , ppReaderSourcePath  :: FilePath
+    -- ^ The file path to the source file
+    , ppReaderPPs         :: [PP]
+    -- ^ Preprocessors being invoked.
+    }
+    deriving (Show, Eq)
+
+-- | A monad in which preprocessor actions happen.
+newtype PPM a = PPM { unPPM :: ReaderT PPReader IO a }
+    deriving (Monad, MonadIO, (MonadReader PPReader))
+
+runPPM :: PPM a -> PPReader -> IO a
+runPPM comp readerData = runReaderT (unPPM comp) readerData
+
 -- | A string which should contain information about why an asset failed to
 -- compile.
 type CompileError = B.ByteString
@@ -47,7 +69,7 @@
     { ppExtension :: Text
     -- ^ The file extension this preprocessor acts upon, eg \"sass\" or
     -- \"hamlet\"
-    , ppAction    :: B.ByteString -> IO (Either CompileError B.ByteString)
+    , ppAction    :: B.ByteString -> PPM (Either CompileError B.ByteString)
     -- ^ Perform the preprocessing.
     }
 
@@ -64,7 +86,7 @@
 
 -- | A collection of preprocessors.
 newtype PPs = PPs { unPPs :: M.Map Text PP }
-    deriving (Show)
+    deriving (Show, Eq)
 
 noPPs :: PPs
 noPPs = PPs M.empty
@@ -98,12 +120,12 @@
     , hbPPs        :: PPs
     -- ^ Preprocessors
     }
-    deriving (Show)
+    deriving (Show, Eq)
 
 -- | All assets in Herringbone are referenced by their logical path. This is
 -- the path to an asset, relative to any of the source directories.
 newtype LogicalPath = LogicalPath { fromLogicalPath :: [Text] }
-    deriving (Show)
+    deriving (Show, Eq)
 
 -- | Create a LogicalPath from a list of Text values. This returns Nothing if
 -- the path would be unsafe (that is, if it contains \"..\"), to prevent
diff --git a/herringbone.cabal b/herringbone.cabal
--- a/herringbone.cabal
+++ b/herringbone.cabal
@@ -1,5 +1,5 @@
 name:                herringbone
-version:             0.0.3
+version:             0.0.4
 synopsis:            A library for compiling and serving static web assets.
 description:         A library for compiling and serving static web assets. For more information, please see <https://github.com/hdgarrood/herringbone>.
 license:             MIT
@@ -25,6 +25,7 @@
                         text,
                         bytestring,
                         containers,
+                        mtl,
                         system-filepath,
                         system-fileio,
                         directory,
@@ -38,7 +39,8 @@
   default-extensions:   OverloadedStrings,
                         TypeSynonymInstances,
                         FlexibleInstances,
-                        RankNTypes
+                        RankNTypes,
+                        GeneralizedNewtypeDeriving
   default-language:     Haskell2010
 
 library
@@ -56,6 +58,7 @@
                         text,
                         bytestring,
                         containers,
+                        mtl,
                         system-filepath,
                         system-fileio,
                         directory,
@@ -66,5 +69,6 @@
   default-extensions:   OverloadedStrings,
                         TypeSynonymInstances,
                         FlexibleInstances,
-                        RankNTypes
+                        RankNTypes,
+                        GeneralizedNewtypeDeriving
   default-language:     Haskell2010