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 original dualism between praetorian law and civil law determined to varying degrees the development of Roman law until the codification of the edict, when the distinction between civil law and praetorian law became meaningless. The Twelve Tables, the earliest Roman collection or set of fundamental rules of customary law, constituted the foundational text of the entire Roman legal system, a model, symbol, and paradigm of statutory law. Lex and plebiscitum were the statutes of the Republican period. The new law that the magistrates introduced through edicts to adapt civil law to new circumstances and needs of the political community was called honorary law. The responsa were the meeting point between the legal science and the legal practice of ordinary life, and they contributed deeply to the development of Roman society.