Chapter 6 discusses a topic within social evolution that has been much debated in biology, namely group or multilevel selection. The idea is that interactions among, and dynamics of, groups contribute to explaining organizational and institutional features of animal and human societies. We address emergence, growth and selection of groups, including their competition, differential growth, conflicts, mergers and takeovers. The chapter examines distinct interpretations of group selection, provides a complete account of group selection mechanisms as identified in the literature, and reviews empirical and experimental evidence. This includes attention for migrant-pool versus propagule types of group formation, non-random assortment, spatial and behavioural population structures, norm-preserving versus norm–diffusing institutions, group splitting and conflict, and non-additive genetic interaction between individuals. In addition, particular features and implications of cultural group selection applying to human groups are considered. Potential applications of group selection thinking aimed at clarifying particular economic, organizational, institutional and policy issues are examined.