During social interactions, people influence each other in a variety of subtle and overt ways—for example, through their tone of voice, nonverbal behaviors, and facial expressions. In this talk, I will describe a particular case of social influence—physiological linkage—which occurs when one person’s physiological response predicts another person’s physiological response at a future time point. First, I will present a theoretical framework for understanding how physiological linkage occurs and the psychological inferences that can be drawn from it. Next, I will describe a set of studies examining physiological linkage alongside dyadic behaviors, group outcomes, and country-level measures of social relationships. Across these studies, I will show that linkage is conditional and occurs when people are socially attuned to one another. Finally, I will discuss the importance of physiological linkage for understanding dyadic and group behavior, as well as implications of these findings for health and well-being.