This article examines the question of criminal liability in terms of the theoretical distinction between justification and excuse. By contrast with German and other continental criminal law systems, the distinction has not played a significant part in the development of criminal law doctrine in common law jurisdictions. Over the past twenty years, however, there has been a growing interest in the benefits of this approach to conceptualising criminal liability, manifested by the considerable literature on justification and excuse and the frequent references to the distinction in judicial decisions and legislative enactments. Although the distinction has been given a great deal of attention in common law countries in recent years, attempts at a systematic classification of criminal law defences on this basis run up against serious difficulties. These difficulties have much to do with the fact that elements of both justification and excuse often appear to overlap in the moral basis of a legal defence. It is argued that, notwithstanding these difficulties, the theory of justification and excuse offers a viable model, which can achieve and maintain coherence among criminal law defences and facilitate understanding and acceptance of criminal law and its presuppositions.