Women's groups are a popular approach to promoting women's and girls' empowerment. Yet, whether and how creating and supporting women's groups and delivering interventions through them offers unique benefits compared to individual-based interventions remains an open question. We review the experimental and quasi-experimental literature on women's livelihoods and financial groups, health groups, and adolescent groups, and analyze the causal mechanisms through which these models improved outcomes for women and girls in low and middle-income countries. We distinguish between mechanisms that leveraged groups as a platform for intervention delivery and mechanisms that leveraged interactions among group members. We conclude that the primary benefit of group models is to offer a platform to reach many women at once with resources, information, and training. Nonetheless, some evidence suggests that group models can achieve positive impacts by fostering or harnessing interactions among group members, which would be harder or impossible to achieve through individual-based interventions. We offer some suggestions regarding the implications of these findings for programming and future research.