This review offers a theoretical framework to account for the effects of peers on youths' physical activity. Our research indicates the following: 1) that the youth are more physically active in the presence of friends and peers than in the presence of family members or when alone, 2) peers and friends increase overweight/obese youths' motivation to be physically active, 3) peers' weight status does not moderate the effect of peers on youths' physical activity, and 4) experiencing negative peer interaction, such as ostracism, decreases physical activity in youth. We propose that the consideration of the peer social context as a contributor to physical (in)activity and maintenance of overweight status may further our understanding of physical and behavioral health trajectories and improve prevention and intervention efforts.