diff --git a/HACKING b/HACKING
index bbfd76dace9f92258be3f1058b5a8022c834a9be..bb553adeb4b9ae4455505311a64bd9d588713b97 100644
--- a/HACKING
+++ b/HACKING
@@ -43,6 +43,14 @@ Perl 5.005_02
 
 The perl version should not matter.
 
+Bison and flex
+--------------
+
+You may need the Gnu tools bison and flex. The standard lex and yacc
+are not flexible enough. You should have bison version 1.25 or later
+and flex version 2.5.4 or later.
+
+
 
 
 Release generation