Emotions, thoughts, and intentions are not simply concepts that live privately in our minds, but rather, affective states emanate from us via multiple channels—voice, posture, facial expressions, and behavior—and can influence those around us. Affect contagion, or the spread of affective states from one person to another, is studied in a variety of ways in the social sciences: sociologists study how happiness is contagious within social networks, social psychologists examine how imitation of affective states influences how we perceive others, and neuroscientists show that observing someone experience pain produces similar neural activation as experiencing the same pain. In this chapter, we present a theory of affect contagion that identifies processes, antecedents, moderators, and consequences. Using peripheral psychophysiology coupled with dyadic data modeling, we review a series of studies exploring social and personality factors that facilitate (or attenuate) affect contagion, specifically closeness, group similarity, status, arousal, and valence. We then extend the question of how situational and personal factors contribute to affect contagion, to speculate on possible social and behavioral consequences such as mutual trust and cooperative performance. Collectively this work has the potential to illuminate the antecedents of affect contagion, and the behavioral consequences for individuals and groups.