In this entry for the Encyclopedia of the Philosophy of Law and Social Philosophy, I present the main themes related with the ontology of law. I make a distinction between two ways of conceiving the question "What is Law?": one focused on the nature of legal facts as institutional, or social facts, another focused on the peculiar character of these facts. I make a distinction between three kinds of institutional concepts relevant for an analysis of the legal domain and put forward a three-dimensional theory of legal ontolog
