Message theory is concerned with directed messages, the messages that are the units of rational communication. In this paper we present an introductory sketch of message theory and show to what extent it can provide a semantics for dialogue. At first glance it might appear from the description given below that message theory provides only for the semantics, and indeed only for a few of the extensional aspects of the semantics. It should become clear, however, that the message-theoretic framework provides for many possibilities beyond those explicitly mentioned in this paper — possibilities for both the semantics and the pragmatics of dialogue.