What are the limits of law? To answer this question, we need criteria that allow to separate the norms of law from other norms. These criteria are elements of the definition of law, they determine the extension of its concept. In a modern system of law such criteria are established by norms that identify the norms that belong to the same system. These norms specify the formal (about the mode of production or the sources) and the material (about the content) conditions of validity of the norms of English law or of Portuguese law, for example, as distinct legal systems and distinct from international law or European law. They belong to the material constitution of each system but do not differentiate conditions of being law from conditions of being English or Portuguese etc. law. The first set of conditions are however necessarily implied. Besides the norms that are recognized as originated by the same constitutional system or by the same system of sources, there are other norms (originated by other systems) that such a system prescribes to its addressees through transitional law, international private law, international law or constitutional change. In these cases, the material conditions of validity of a norm are usually reduced to the conditions of its being law, according to the first system. The concepts of different types of constitutional policy (ordre publique constitutionelle) and of ius cogens define in these cases the limits of law. The paper gives a survey of such concepts and discusses their philosophical relevance.