Laws are part of legal systems; a particular law is a law only if it is part of American law or French law or some other legal system. Legal philosophers have persistently attempted to explain why we think of laws as forming legal systems, to evaluate the merits of this way of thinking about the law and to make it more precise by explicating the features that account for the unity of legal systems. Various theories have been suggested but none has been accepted as completely satisfactory, and the continuing debate owes much to the intricacy of the problems involved. This intricacy is also due to a considerable cloudiness in the understanding of the problems themselves. I will attempt, through this Article, to clarify the nature of the problem of the unity of municipal legal systems. My primary aim is not to answer all the relevant questions, but rather to formulate them more precisely, for I believe that in philosophy a clear conception of a problem is half the way to its solution.