Few communication behaviors are more consequential to the development and maintenance of close relationships than the expression of affection. Indeed, people often use affectionate gestures to initiate or accelerate relationship development. In contrast, the absence of affection in established relationships frequently coincides with relational deterioration. This text explores the scientific research on affection exchange that has emerged from the disciplines of communication, social and clinical psychology, family studies, psychophysiology, sociology, nursing, and behavioral health. Specific points of focus include the individual and relational benefits - including health benefits - of affectionate behavior, the significant detriments associated with lacking sufficient affection, and the risks of expressing affection. It also discusses the primary social and cultural influences on affection exchange, critiques principal theories and measurement models, and offers suggestions for future empirical research.