This collection is a pioneering effort to bring together in fruitful interaction the two dominant perspectives on social rules. One, shared by philosophers, lawyers, anthropologists, and sociologists, directly invites formalization by a logic of rules. The other, originating with economists, emphasizes cost considerations and invites mathematical treatment, often in game-theoretical models for problems of coordination--models that some philosophers have taken up as well.Each perspective is represented by new and recent work that moves this important topic toward increased conceptual precision and deeper insight. As a whole, the collection strikes a balance between historical illustrations and theoretical argument, offering in both a rich body of suggestions for further work.