How is it that people with different and often conflicting interests can band together, overcome coordination problems, and create stable institutions that regulate the interactions among members of the group? Explaining Social Institutions leads us significantly closer to understanding how such institutions come to be. Much of the work being done under the rubric of "new institutionalism" focuses on how institutions shape social, economic, and political outcomes. This emphasis on influence has provided students of economics, political science, and political economy with surprisingly little theory to account for the origins of such institutions. Yet without understanding how institutions form and consequently develop influence, much of the other work lacks context. The contributors fill this void by utilizing a variety of perspectives and theoretical approaches. The twin focus of these articles on the origins of institutions and the development of institutional influence yield innovative and suggestive outcomes. Topics range from the framing of the United States Constitution to debate over the Senate at the Federal Convention; from equilibrium and social institutions to democratic stability. Contributors include Randall Calvert, Jon Elster, Avner Greif, Jack Knight, Paul Milgrom, Douglass North, William Riker, Norman Schofield, Itai Sened, and Barry Weingast. Jack Knight is Assistant Professor of Political Science, Washington University, St. Louis. Itai Sened is Assistant Professor of Political Science, Tel Aviv University.