Just to dump in two cents.... I was receiving the same error when moving a webapp from Tomcat standalone to JBoss 4.0.4 GA. When looking at the existing web.xml I noticed that some properties that were set using net.sourceforge.myfaces. Recalling from some JBoss JSF documentation that the default JSF implementation was provided by Apache, I looked into which library Tomcat (embedded) was using - and behold it was Apache's implementation. Now I would think that having the net.sourceforge implementation in my WEB-INF would prompt Tomcat to use that impl, but it didn't. When I replaced the default Apache impl with the net.sourceforge everything was fine. How to change jsf impl: Section 9.9 of: http://docs.jboss.org/jbossas/jboss4guide/r4/html/ch9.chapt.html (thanks to a previous post for providing this URL) Afore mentioned tag lib issue might still be an issue but for me it was just the JSF implementation. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3959572#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...