diff --git a/include/tclap/CmdLine.h b/include/tclap/CmdLine.h
index 1d80b94..b736346 100644
--- a/include/tclap/CmdLine.h
+++ b/include/tclap/CmdLine.h
@@ -46,6 +46,7 @@
 #include <iostream>
 #include <iomanip>
 #include <algorithm>
+#include <stdlib.h> // Needed for exit(), which isn't defined in some envs.
 
 namespace TCLAP {