Sorry for my late answer :). Those transactions are independent from each other. But step by step: During the existence of an Extended Persistence Context there can be multiple Java Transactions (JTA). And each Java Transaction will encapsulate write accesses to the database in a database transaction, if necessary (but I'm not sure about that). Don't know what about read accesses ;). Usually (i.e. in a Stateless or Stateful Bean with default transaction stuff) changes to entities get written to the database at the end of a method. And those changes are encapsulated in a transaction (which is created by the EJB framework). You might even write changes explicitely to the database with em.flush() before a method ends. Then the SQL-Statements are created immediately and submitted to the database, so you can react to exceptions if necessary (e.g. when violating db constraints). But I must confess, I am not 100% sure at which point in time a transaction starts and ends. My imagination is like this: - before methode starts: em.getTransaction().start(). - after method ends: em.getTransaction().commit() (or rollback() if setRollbackOnly() was called or a RuntimeException occured). - and whenever there are Java transactions running, a database transaction is started before the first db access (e.g. set autocommit=0 in MySQL or begin tran in MS SQL). And after writing was finished, a database commit (or rollback) is called. GAVIN? Could you take as on a ride into the deep misterys of the Java Transaction API and its interactions with the EntityManager here? Only a short comment wether my findings are correct :) Plus some more questions I have: - If a rollback was set (setRollbackOnly() or RuntimeException) there won't be any SQL statements sent to the database at the end of a method. If you called em.flush() before, then statements were already created and only then you need a database rollback, right? - Ist there a SQL commit called at the end of em.flush()? If yes, and if there are e.g. constraints violated, the database rolls back the changed stuff automatically and throws an exception, right? - How does the EntityManager know which entities changed? Does he hold a copy of every managed entity in memory and compare it to the actual entities, field by field?!? Or are there some interceptors doing some observer stuff on the entity? View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3990445#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...