• Synchronous message passing means that each time a message is sent, control passes from the sending object to the receiving object. Information travels back to an originating object through a chain of nested operation calls. Because return is implicit in an operation call, this path is not shown on the diagram.

• Asynchronous message passing means that the sending and receiving objects have separate threads of control that do not synchronize during message passing. When communicating asynchronously an object sends messages without waiting for replies.