This chapter addresses the origin and development of Roman legal sources — that is, the methods and procedures for establishing new legally binding rules, standards, and norms. The source of a legal norm gives it ultimate validity. Legal sources can be classified in many different ways. In this chapter the classification will be based on the distinction between power (potestas) and authority (auctoritas) because this is probably the most comprehensive way to understand the legal implications of Rome’s transformation from the Republic to the Principate and then to the later Empire. During the Republic, the legal sources produced by individuals or institutions vested with power (popular assemblies and magistrates) were basically the statutes (leges and plebiscita) and the edicts of the magistrates. On the other hand, the legal sources produced by individuals or institutions vested with moral authority (auctoritas) were the legal opinions (responsa) of the jurists and the resolutions of the Senate (senatus consulta). The Principate brought a major alteration in the system of legal sources when the authority of the jurists and senators was subordinated to the power of the emperor. The emperor granted legislative power to the Senate. As result, senatorial resolutions became, in practice, legislation, that is, a source of coercive power instead of a source of moral authority. The responsa of the jurists, however, were submitted to imperial influence, opening the door to imperial rescripts, elaborated by the imperial chancellery under the direct control of the emperor. During the later Empire, all sources of law ended up being mere expressions of the imperial power (imperial constitutions). The authority of jurists and senators ceased to be a constitutional limit on imperial power. Moral authority and imperial power were entirely identified in the person of the emperor, whose will had the force of a statute.