What duty do we have to stop others from doing wrong? The question is intelligible in almost any culture, but few seek to answer it in a rigorous fashion. The most striking exception is found in the Islamic tradition where "commanding right and forbidding wrong" is a central moral tenet. Michael Cook's comprehensive and compelling analysis represents the first sustained attempt to chart the history of Islamic reflection on this obligation and to explain its relevance for politics and ideology in the contemporary Islamic world.