Gratitude is an acknowledgment that we have received something of value from others. It arises from a posture of openness to others, where we are able to gladly recognize their benevolence. Societies through the ages have long extolled the benefits of gratitude, and classical writings have deemed it the “greatest of the virtues.” But only recently has psychological theory and research on gratitude begun to catch up with philosophical commendations. This chapter begins by reviewing research on gratitude and positive human functioning. It explores the mechanisms by which gratitude enhances well-being and considers at several explanations and evaluates the empirical evidence for each. The chapter then establishes an agenda for the future by considering some ways in which the scientific field of gratitude can be advanced.