Leaders and diplomats frequently invoke the notion of "legitimacy" -- indeed, debate over the recent wars in Kuwait, Kosovo, Afghanistan, and Iraq was dense with claims and counterclaims about the legitimacy of military action. But in the scholarly world, the meaning and significance of legitimacy remain elusive and understudied. Clark provides one of the most systematic and historically informed accounts of international legitimacy to appear in many years. The abstract meaning of legitimacy is not controversial, but ambiguities emerge when the focus turns to its delineation, its source, and its importance. Clark first argues that norms of legitimacy matter in the real world -- the mere fact that norms of acceptable behavior are acknowledged by governments means that there is something akin to an "international society" lurking in the background of interstate relations. He then argues that actual substantive standards of legitimacy evolve over time but get fixed in place during peace settlements after major wars. Unfortunately, on the most interesting question -- how the search for legitimacy shapes and constrains powerful states -- Clark is not entirely successful, although he correctly observers that in today's world of democracies, international legitimacy is both more important and more difficult to achieve than ever.