Section 15.4.8 of the EJB 2.0 specification states that "If the message-driven bean uses container managed transaction demarcation, message acknowledgment is handled automatically as a part of the transaction commit." Further, in section 18.104.22.168, it says "If the onMessage method does not successfully complete or the transaction is rolled back by the Container, JMS message redelivery semantics apply." The JMS message redelivery semantics ensures that if a failure occurs while executing the onMessage() method, the message is automatically redelivered. The bottom line is that when you use CMT, the container is responsible for acknowledging the message. If the transaction fails, the container should redeliver the message until is goes through. Certain vendor-specific optimizations and strategies may apply, though.