diff --git a/tests/caasdriver.nim b/tests/caasdriver.nim
index b36b6f5b73..f319512b9f 100644
--- a/tests/caasdriver.nim
+++ b/tests/caasdriver.nim
@@ -1,5 +1,27 @@
 import osproc, streams, os, strutils, re
 
+## Compiler as a service tester.
+##
+## This test cases uses the txt files in the caas/ subdirectory.
+## Each of the text files inside encodes a session with the compiler.
+## The first line indicates the main project file. Lines starting with '>'
+## indicate a command to be sent to the compiler and the lines following a
+## command include checks for expected or forbidden output (! for forbidden).
+##
+## You can optionally pass parameters at the command line to modify the
+## behaviour of the test suite. By default only tests which fail will be echoed
+## to stdout. If you want to see all the output pass the word "verbose" as a
+## parameter.
+##
+## If you don't want to run all the test case files, you can pass any substring
+## as a parameter. Only files matching the passed substring will be run. The
+## filtering doesn't use any globbing metacharacters, it's a plain match.
+##
+## Example to run only "*-compile*.txt" tests in verbose mode:
+##
+##   ./caasdriver verbose -compile
+
+
 type
   TNimrodSession* = object
     nim: PProcess