diff --git a/docs/ispc.txt b/docs/ispc.txt
index 4cfefdd1..53c2bfa2 100644
--- a/docs/ispc.txt
+++ b/docs/ispc.txt
@@ -57,6 +57,7 @@ Contents:
 
   + `Basic Command-line Options`_
   + `Selecting The Compilation Target`_
+  + `The Preprocessor`_
 
 * `The ISPC Language`_
 
@@ -288,10 +289,6 @@ with application code, enter the following command
 
    ispc foo.ispc -o foo.o
 
-``ispc`` automatically runs the C preprocessor on your input program before
-compiling it.  (This functionality can be disabled with the ``--nocpp``
-command-line argument.)
-
 Basic Command-line Options
 --------------------------
 
@@ -380,6 +377,23 @@ use ``--target=sse2``.  (As with the other options in this section, see the
 output of ``ispc --help`` for a full list of supported targets.)
 
 
+The Preprocessor
+----------------
+
+``ispc`` automatically runs the C preprocessor on your input program before
+compiling it.  Thus, you can use ``#ifdef``, ``#define``', and so forth in
+your ispc programs (This functionality can be disabled with the ``--nocpp``
+command-line argument.)
+
+Three preprocessor symbols are automatically defined before the
+preprocessor runs.  First, ``ISPC`` is defined, so that it can be detected
+that the ``ispc`` compiler is running over the program.  Next, a symbol
+indicating the target instruction set is defined.  With an SSE2 target,
+``ISPC_TARGET_SSE2`` is defined; ``ISPC_TARGET_SSE4`` is defined for SSE4,
+and ``ISPC_TARGET_AVX`` for AVX.  Finally, ``PI`` is defined for
+convenience, having the value 3.1415926535.
+
+
 The ISPC Language
 =================
 
diff --git a/module.cpp b/module.cpp
index cf8b2836..2881c810 100644
--- a/module.cpp
+++ b/module.cpp
@@ -909,12 +909,27 @@ Module::execPreprocessor(const char* infilename, llvm::raw_string_ostream* ostre
 
     clang::PreprocessorOptions &opts = inst.getPreprocessorOpts();
 
-    //Add defs for ISPC and PI
+    // Add defs for ISPC and PI
     opts.addMacroDef("ISPC");
     opts.addMacroDef("PI=3.1415926535");
 
+    // Add #define for current compilation target
+    switch (g->target.isa) {
+    case Target::SSE2:
+        opts.addMacroDef("ISPC_TARGET_SSE2");
+        break;
+    case Target::SSE4:
+        opts.addMacroDef("ISPC_TARGET_SSE4");
+        break;
+    case Target::AVX:
+        opts.addMacroDef("ISPC_TARGET_AVX");
+        break;
+    default:
+        FATAL("Unhandled target ISA in preprocessor symbol definition");
+    }
+
     for (unsigned int i = 0; i < g->cppArgs.size(); ++i) {
-        // Sanity Check, should really begin with -D
+        // Sanity check--should really begin with -D
         if (g->cppArgs[i].substr(0,2) == "-D") {
             opts.addMacroDef(g->cppArgs[i].substr(2));
         }