Neither modern states nor small farmers in remote areas of poor countries can appeal to authoritative hierarchies to enforce rules governing their relations with one another. Both must finds ways to reach agreements that can be maintained through reciprocity if they are to avoid the tragedy of the commons from which they cannot extract themselves. This book explores the conditions for and possibilities of reaching such agreements. It develops a theoretical understanding of co-operation and discord at local and global levels, focusing on two of the key variables that affect outcomes - the number of actors, and the degree of heterogeneity between them. The book provides a broad-ranging introduction to the theory and theoretical issues involved, combining this with a detailed review of research evidence on how agreements to co-operate are established and maintained.