Over the past 20 years group dynamics-based interventions have been used to successfully increase physical activity. However, the literature is less clear on the underlying mechanisms of effectiveness. That is, what makes these group dynamics interventions work? We conducted a systematic review to identify studies that used different group dynamics strategies to promote physical activity. Seventeen studies were identified and were coded by two raters to determine the degree to which group dynamics strategies were used, the format of the programs, and any analytic procedures used to determine the causal mechanisms underlying intervention effectiveness. The results of the coding indicated that while there is no standard package of group dynamics strategies being applied across the literature- and regardless of the breadth of the underlying theory or the structure of the programs- the effect on physical activity is robust. However, few studies explicitly measured potential causal mechanisms and even fewer completed the necessary analysis to detect mediation. We concluded that future research on developing a unified theory for group dynamics with appropriate measurement tools is necessary to further enhance the effects of group dynamics on physical activity promotion.