Collective action for sustainable management among resource-dependent populations has important policy implications. Despite considerable progress in identifying factors that affect the prospects for collective action, no consensus exists about the role played by heterogeneity and size of group. The debate continues in part because of a lack of uniform conceptualization of these factors, the existence of non-linear relationships, and the mediating role played by institutions. This article draws on research by scholars in the International Forestry Resources and Institutions (IFRI) research network which demonstrates that some forms of heterogeneity do not negatively affect some forms of collective action. More importantly, IFRI research draws out the interrelations among group size, heterogeneity, and institutions. Institutions can affect the level of heterogeneity or compensate for it. Group size appears to have a non-linear relationship to at least some forms of collective action. Moreover, group size may be as much an indicator of institutional success as a precondition for such success.Funding from the Ford Foundation, the MacArthur Foundation, and the National Science Foundation (SBR 9521918) is gratefully acknowledged. The authors thank Clark Gibson and two anonymous reviewers for thoughtful comments, Lauren Persha for research assistance, and Patty Lezotte for her careful editing. Development and Change 35(3): 435-461 (2004).