Following up her landmark work On Social Facts, this collection of essays by noted social philosopher Margaret Gilbert develops and deepens her theory of social groups as 'plural subjects.' She asks, how far can our rationality take us when we pursue our personal goals? What does it mean to be a member of a group? Does group membership involve obligations and rights, and, if so, how? Gilbert argues that, in order to understand the social dimensions of human life, we must go beyond the prevailing 'game theoretic' picture of people acting as independent individuals, to incorporate their situation as group members, or plural subjects bound together by joint commitments. Her new theory of obligation will be of interest to scholars engaged in empirical research as well as to philosophers and social and political theorists.