I have been trying to upgrade to V5M4 with little success. I have all the Drools libraries in a single Eclipse plug-in. I have both API and Core libraries which is certainly confusing Eclipse and any plug-ins that are dependent on it. I have also tried just using Core and deleting API from my workspace but it appears that some of the classes may have been built against the classes in API, specifically: FactHandle accountHandle = session.insert(account); throws a compiler error of The project was not built since its build path is incomplete. Cannot find the class file for org.drools.runtime.rule.WorkingMemoryEntryPoint. Why calling insert on StatefulSession which is in core would be complaining about an API class is beyond me unless Core was compiled against API and for some reason the core class had a reference to WorkingMemoryEntryPoint from the API library. Again this is in an Eclipse plug-in world so you may not face these problems depending on your runtime environment. I think the only way you can be sure is to migrate and try in your own runtime. Brody bach wrote: -- View this message in context: http://www.nabble.com/which-libraries-to-use-api-or-core-tp21014726p21015... Sent from the drools - user mailing list archive at Nabble.com.