This book examines the issue of rational cooperation, especially cooperation between people with conflicting moral commitments. The first part considers how the two main aspects of cooperation - the choice by a group of a particular cooperative scheme and the decision by each member to contribute to that scheme - can be understood as guided by reason. The second part explores how the activity of reasoning itself can take a cooperative form. The book is distinctive in offering an account of what people can accomplish by reasoning together, of the role of deliberation in democratic decision making, and of the negotiation of the proper use of concepts. Presenting for the first time a detailed analysis of the general problem of cooperation and collective reasoning between people with different moral commitments, this book will be of particular interest to philosophers of the social sciences and to students in political science, sociology and economics.