There has been an explosion of interest on happiness and positive emotion in both the scientific literature and the popular press. While positive emotion is generally considered a source of good outcomes, recent scientific work in psychology has highlighted the ways in which positive emotion facilitates the pursuit of important goals, contributes to vital social bonds, broadens our scope of attention, and increases psychological and physical well-being. But this wave of interest in positive psychology has to date neglected another important possibility regarding positive emotion--that it may, under certain conditions, be maladaptive. Here, Gruber and Moskowitz propose that the field is now ripe to consider the costs, and not just the benefits, of positive emotion. This book offers the first comprehensive exploration of this phenomenon. It offers a comprehensive summary of current theoretical and empirical work on positive emotion and provides empirical examples of the 'light side' or adaptive benefits of positive emotion according to the degree, context (health, social relationships, coping), and type of adaptive outcome. It also provides empirical examples of the 'dark side' or maladaptive aspects of positive emotion organized according to the degree, context, type and reasons for pursuing positive emotion in healthy and clinical populations. It discusses therapeutic applications regarding how to cultivate and foster healthy positive emotion, and suggests future research to better understand the nature of positive emotion.