This book, first published in 2009, examines the ways in which reasonable people can disagree about the requirements of political morality. Christopher McMahon argues that there will be a 'zone of reasonable disagreement' surrounding most questions of political morality. Moral notions of right and wrong evolve over time as new zones of reasonable disagreement emerge out of old ones; thus political morality is both different in different societies with varying histories, and different now from what it was in the past. McMahon explores this feature of his theory in detail and traces its implications for the possibility of making moral judgments about other polities, past or present. His study sheds light on an important and often overlooked aspect of political life, and will be of interest to a wide range of readers in moral and political philosophy and in political theory.