The essay seeks to make contributions to the clarification of the conceptual relation between law and politics. It characterizes law as an institutionalized and normative social practice that makes authority claims on its participants. On this basis, legal institutions are defined as institutions that systematically seek to influence human conduct by providing authoritatively binding practical reasons. The essay claims that the elucidation of the conceptual features of legal institutions touches upon a series of issues of justification that belong to the realm of political philosophy. This makes concepts like political institution and political obligation relevant for conceptual legal theory. After an analysis of the concept of political institution, the essay claims that the concept of legal institution and the concept of political institution have the same applications. This conclusion is used in support of the main thesis of essay: legal institutions are to be treated as political institutions in conceptual legal theory. The essay also examines whether the conceptual framework outlined here can be compatible with a viable notion of political communities. The essay makes an attempt to clarify the relevance of the main thesis in respect to legal reasoning; it insists that the position taken here is unlikely to lead to some radical reorientation of legal reasoning.