When people produce a discourse, what needs are they responding to when they make it coherent, and what form does this coherence take? In this paper, it is argued that coherence can be characterized in terms of a set of “coherence relations’’ between segments of a discourse. It is shown, from an abstract description of the discourse situation, that these relations correspond to the kinds of communicative work that needs to get done in discourse. In particular, four requirements for successful communication are isolated: the message itself must be conveyed; the message must be related to the goals of the discourse; what is new and unpredictable in the message must be related to what the listener’s inference processes toward the full intended meaning of the message. Corresponding to each requirement is a class of coherence relations that help the speaker satisfy the requirements. The coherence relations in each class are discussed and defined formally. Finally, a fragment of a conversation is analyzed in detail to illustrate the problems that face a speaker in trying to satisfy these requirements, and to demonstrate the role the coherence relations play in the solution.