We argue that current plan-based theories of discourse do not by themselves explain even simple task-oriented dialogues. The purpose of this paper to show how a number of difficult-to-explain features of these dialogues follow from the joint or team nature of the underlying task. Specifically, the paper formally defines the concept of a joint intention and we argue that the conversants in a task-oriented dialogue jointly intend to accomplish the task. From this basis, the paper derives the goals underlying the pervasive use of confirmations, elaborations, requests for clarification, and temporal discourse markers observed in a recent experiment. We conclude with a discussion on generalizing the analysis presented to characterize dialogue itself as a joint activity.