This is the first systematic philosophical and conceptual study of the notion of a social practice. Raimo Tuomela explains social practices in terms of the interlocking mental states of the agents; he shows how social practices (for example customs and traditions) are 'building blocks of society'; and he offers a clear and powerful account of the way in which social institutions are constructed from these building blocks as established, interconnected sets of social practices with a special new social status. His analysis is based on the novel concept of shared 'we-attitudes', which represent a weak form of collective intentionality, and he makes instructive connections to major topics and figures in philosophy and the social sciences. His book will be of interest to a wide range of readers in philosophy of mind, philosophy of social science, psychology and sociology, and artificial intelligence.