Although the argumentation justifying decisions in particular cases has always been central to AI and Law, it has recently become a burning issue as black box machine learning approaches become prevalent. In this paper we review the understanding of legal argument that has been developed in AI and Law, and indicate the most appropriate ways in which Machine Learning approaches can contribute to legal argument. We identify some key questions that must be explored to provide acceptable explanations for legal ML systems. This provides the context and directions of our current research project.