The ritual animal longs to belong. Rituals are a way of defining the boundaries of social groups and binding their members together. The ritual modes theory set out in this book seeks to unravel the psychology behind these processes, and to explain how ritual behaviour evolved, including how different modes of ritual performance have shaped global history over many millennia. Testing the theory has meant designing experiments run with children in psychology labs and on remote Pacific islands, gathering survey data with armed insurgents in the Middle East and Muslim fundamentalists in Indonesia, monitoring heart rate and stress among football fans in Brazil, and measuring changes in the brain as people observe traditional Chinese rituals in Singapore. The results of all this research point to new ways of addressing cooperation problems: from preventing violent extremism to motivating action on the climate crisis. Although this book is about the role of ritual in the evolution of social complexity, more broadly it models a new approach to the science of the social—an approach that is driven by real-world observation but grounded in the cognitive and evolutionary sciences. More ambitiously still, it shows how cumulative theory building can be used to deliver practical benefits for society at large, perhaps even addressing problems on a global scale by harnessing the formidable cohesive and cooperative capacities of the ritual animal.