A message is a communication between objects that conveys information with the expectation that action will ensue. The receipt of a message is one kind of event. The message is drawn as a horizontal line from one object lifeline to another. The message may also be drawn from and to the same object, representing a message from an object to itself.

A message is shown as a line from the sender MessageEnd to the receiver MessageEnd. The line must be such that every line fragment is either horizontal or downwards when traversed from send event to receive event. The send and receive events may both be on the same lifeline.