Challenging the prevailing understanding of the authority of law, Daniel Mark offers a theory of moral obligation that is rooted both in command and in the law's orientation to the common good. When and why do we have an obligation to obey the law? Prevailing theories in the philosophy of law, starting with the work of H. L. A. Hart and Joseph Raz, fail to provide definitive answers regarding the nature of legal obligation. In this highly original and effective new work, Daniel Mark argues that there is a prima facie moral obligation to obey the law simply because it is the law. In Mark's view, the best concept of law-one that allows for the possibility of justified authority and obligation-defines law as a set of commands oriented to the common good. Legal obligation, he proposes, shares defining features with moral obligation and with religious obligation while aligning wholly with neither. This philosophically coherent view of legal obligation offers a viable framework for analyzing important and seemingly paradoxical puzzles about the law, such as why civil disobedience is punished as lawbreaking or why war-crimes trials for legal but immoral acts present a moral quandary. By reconciling the concept of law as command with the role of law in promoting the common good, The Nature of Law provides an original and important scholarly contribution to the fields of legal philosophy and political thought.