A large number of bird species live in stable groups, and this sets the scene for complex social behaviours, such as co-operative breeding. The vast majority of groups consist of families which arise when young postpone dispersal and remain with their parents beyond independence. However, the factors selecting for the evolution of families and thus also co-operative breeding among birds, are still a challenging puzzle. The currently accepted key explanation for the evolution of families and co-operative breeding focuses on dispersal constraints. While constraints successfully explain within-population dispersal decisions, they fail as an ultimate explanation because offspring in the majority of species face some sort of dispersal constraint, yet still disperse promptly. Recent alternative explanations focus on the role of philopatry and nepotism, and emphasise a key role of life-history for the evolution of families. In this book the authors first review where prolonged family associations are found, and then present a typology of family living to show the diversity of territorial systems and social systems in which families are found.