All societies are differentiated by age. But in some, this differentiation takes the form of institutionalized, formally graded age classes, the members of which share an assigned 'structural' age, if not necessarily the same physiological age. The nature of formal age group systems has become one of the classic issues in modern social anthropology, although until now there has been no comprehensive explication of these complex forms of social organization. In this book, Bernardo Bernardi, one of the pioneers of the anthropological study of age class systems, provides a way of making sense of the diversity of such systems by analysing cross-culturally their common features and the pattern of their differences, and showing that they serve a general purpose for the organization of society and for the distribution and rotation of power.