Margaret Gilberts A Theory of Political Obligation is an incredibly rich work on an important topic. Gilbert has written a series of influential and important articles on the nature of social groups and joint action, and all of her work in those articles is impressively synthesised and developed here into what is surely one of the most innovative and intriguing accounts of political obligation currently on offer. Her book repays careful study, and any adequate reply would itself have to take the form of a book.