The rapidly growing and evolving literature on the social environment and its influence on health outcomes currently lacks a clear taxonomy of dimensions of the social environment and the differing mechanisms through which each influences health-related behavior. This paper identifies five dimensions of the social environment—social support and social networks, socioeconomic position and income inequality, racial discrimination, social cohesion and social capital, and neighborhood factors—and considers each in the context of physical activity to illustrate important differences between them. Increasing the specificity of terminology and methods in social environmental research on health will enable more systematic inquiry and accelerate the rate of scientific discovery in this important area.