How can the stunning diversity of social systems and behaviours seen in nature be explained? Drawing on social evolution theory, experimental evidence and studies conducted in the field, this book outlines the fundamental principles of social evolution underlying this phenomenal richness.To succeed in the competition for resources, organisms may either 'race' to be quicker than others, 'fight' for privileged access, or 'share' their efforts and gains. The authors show how the ecology and intrinsic attributes of organisms select for each of these strategies, and how a handful of straightforward concepts explain the evolution of successful decision rules in behavioural interactions, whether among members of the same or different species. With a broad focus ranging from microorganisms to humans, this is the first book to provide students and researchers with a comprehensive account of the evolution of sociality by natural selection.