diff --git a/README.md b/README.md
index e3d0c88..b1976ff 100644
--- a/README.md
+++ b/README.md
@@ -44,12 +44,13 @@ path/to/file.sv` will output the converted file to `stdout`.
 sv2v [OPTIONS] [FILES]
 
 Common flags:
-  -e --exclude=CONV     conversion to exclude (always, interface, logic); can
-                        be specified multiple times
-  -i --incdir=DIR       add directory to include search path
-  -? --help             Display help message
-  -V --version          Print version information
-     --numeric-version  Print just the version number
+  -e --exclude=CONV         conversion to exclude (always, interface, logic);
+                            can be specified multiple times
+  -i --incdir=DIR           add directory to include search path
+  -d --define=NAME[=VALUE]  define value for compilation
+  -? --help                 Display help message
+  -V --version              Print version information
+     --numeric-version      Print just the version number
 ```
 
 
diff --git a/src/Job.hs b/src/Job.hs
index fc29fdf..cf9f3d3 100644
--- a/src/Job.hs
+++ b/src/Job.hs
@@ -19,6 +19,7 @@ data Job = Job
     { exclude :: [Exclude]
     , files :: [FilePath]
     , incdir :: [FilePath]
+    , define :: [String]
     } deriving (Show, Typeable, Data)
 
 defaultJob :: Job
@@ -29,6 +30,7 @@ defaultJob = Job
             ++ "; can be specified multiple times")
     , files = def &= args &= typ "FILES"
     , incdir = def &= typDir &= help "add directory to include search path"
+    , define = def &= typ "NAME[=VALUE]" &= help "define value for compilation"
     }
     &= program "sv2v"
     &= summary "sv2v v0.0.1, (C) Zachary Snow 2019, Tom Hawkins, 2011-2015"
diff --git a/src/sv2v.hs b/src/sv2v.hs
index 71f7b89..a1fd27e 100644
--- a/src/sv2v.hs
+++ b/src/sv2v.hs
@@ -7,16 +7,24 @@
 import System.IO
 import System.Exit
 
-import Job (readJob, files, exclude, incdir)
+import Data.List (elemIndex)
+import Job (readJob, files, exclude, incdir, define)
 import Convert (convert)
 import Language.SystemVerilog.Parser
 
+splitDefine :: String -> (String, String)
+splitDefine str =
+    case elemIndex '=' str of
+        Nothing -> (str, "")
+        Just idx -> (take idx str, drop (idx + 1) str)
+
 main :: IO ()
 main = do
     job <- readJob
     -- parse the input file
     let includePaths = incdir job
-    asts <- mapM (parseFile includePaths []) (files job)
+    let defines = map splitDefine $ define job
+    asts <- mapM (parseFile includePaths defines) (files job)
     let ast = concat asts
     -- convert the file
     let ast' = convert (exclude job) ast