The last two decades have seen an explosion of scientific research into everyday affective experience. This book offers a comprehensive overview of current knowledge in this area. With attention to basic theoretical and measurement issues, the book examines short-term fluctuations in mood that result from such factors as stress, social activity, exercise, and weather, as well as from endogenous cycles or rhythms. Long-term individual differences in temperament and emotionality are then explored, and recent theory and evidence on happiness is presented. The final chapters address how individual differences in temperament are implicated in health and psychopathology.