Margaret Gilbert is a leading contributor to the philosophy of the social world. Her first book, On Social Facts (1989) presents a novel, unified approach to a wide range of familiar social phenomena including social groups, group languages, and social conventions. This approach is presented in the context of reflections on the important theories of sociologists Max Weber, Emile Durkheim, and Georg Simmel, and the philosophers Wittgenstein, Saul Kripke, and David Lewis. In subsequent writings Margaret Gilbert has continued further to develop and apply the ideas first introduced in On Social Facts.The key concept in her work is the concept of joint commitment. As she understands it, joint commitment is what transforms you and me into "we". One way to create a joint commitment is to make an agreement with someone, but it is not the only way. Her newest essay collection---Joint Commitment: How We Make the Social World (2013)---displays how many aspects of our lives can be illuminated by appeal to joint commitment. Topics range from marital love to the unity of a group of nations such as the European Union, from the nature of command authority to the nature of agreements and promises. Earlier collections of her essays are Living Together (1996), Sociality and Responsibility (2000), and Marcher Ensemble (in French)(2003). Living Together includes the much referenced article "Walking Together: A Paradigmatic Social Phenomenon", also included in Marcher Ensemble.In her book A Theory of Political Obligation: Membership, Commitment, and the Bonds of Society (2006) Margaret Gilbert uses her theory of social groups to develop a novel approach to a longstanding problem in political philosophy---the problem of political obligation. The problem, in brief, is: do I have a special obligation to obey the laws of my own country in particular? Many people have the sense that the answer is positive, but the philosophical consensus to date has been that it is negative. Gilbert argues that the conditions under which a positive answer makes sense are not as restricted as has been assumed in the negative camp. In particular, the "social contract" does not have to take the form of an explicit agreement but, rather, a joint commitment that can arise without the explicitness of such an agreement. The paperback (2008) contains a number of small clarifying revisions.A central aspect of joint commitment, Gilbert has argued, is that it grounds obligations and rights for the parties, where obligations in the relevant sense are not and do not comprise moral requirements. She is currently at work on a book on rights that develops this point in the context of contemporary rights theory.