Can someone please tell me the difference between these in plain english? I cannot find a clear distinction between them, dispite having reading that chapter several times. Is an EntityManager a PersistenceContext? "A persistence unit maps a fixed set of classes to a relational database." This is a quote from the Enterprise Javabeans 3.0 book. So what's the difference between a PersistenceUnit and a PersistenceContext? I'm particularly interesting in learning when I would use different configurations of these and when to use a particular configuration For example, would I have one EntityManager with two PersistenceContexts because I need to manage my entity objects across two databases? Thanks for any help, Michael. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4057525#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...