In this chapter, we provide evidence of the important influence social factors have on positive human development within a physical activity context. Specifically, we discuss empirical and conceptual findings for three main areas of research. First, we examine the literature regarding social support - the types and sources of social support, outcomes related to social support, how social support positively impacts such outcomes, and social support in special populations. Second, we summarize literature with respect to a specific type of social influence: cohesion. In this section we present evidence of the link between cohesion and adherence as well as other outcomes, and provide insight into teambuilding as a method to enhance cohesion and subsequent outcomes. The last topic we discuss is group-related factors in exercise. We summarize research on two types of exercise leaders (group fitness instructors and personal trainers), and discuss important qualities of the exercise leader that have been shown to be influential (leadership style, experience, and physical characteristics). In addition, we examine literature on characteristics of the exercise group (i.e., co-exercisers' characteristics, group atmosphere, and group size). We have also included a section on practical implications regarding social factors in a physical activity context. We conclude the chapter with key points that highlight future directions in the research area of social factors in exercise settings.