When is it possible for one person to persuade another to change her action? We take a mechanism design approach to this question. Taking preferences and initial beliefs as given, we introduce the notion of a persuasion mechanism: a game between Sender and Receiver defined by an information structure and a message technology. We derive necessary and sufficient conditions for the existence of a persuasion mechanism that strictly benefits Sender. We characterize the optimal mechanism. Finally, we analyze several examples that illustrate the applicability of our results.