When people are in a certain mood, whether elated or depressed, that mood is often communicated to others. When we are talking to someone who is depressed it may make us feel depressed, whereas if we talk to someone who is feeling self-confident and buoyant we are likely to feel good about ourselves. This phenomenon, known as emotional contagion, is identified here, and compelling evidence for its effects is offered from a variety of disciplines--social and developmental psychology, history, cross-cultural psychology, experimental psychology, and psychopathology. The authors propose a simple mechanism to account for the process of contagion. They argue that people, in their everyday encounters, tend automatically and continuously to synchronize with the facial expressions, voices, postures, movements, and instrumental emotional behaviors of others. Emotional experiences are affected, moment-to-moment, by the feedback from such mimicry. In a series of orderly chapters, the authors provide observational and laboratory evidence to support their propositions. They then offer practical suggestions for clinical psychologists, physicians, husbands and wives, parents, and professionals who wish to become better at shaping the emotional tone of social encounters.