The Roman Empire systematically recognised alien polities in provincial landscapes and allowed them local powers of jurisdiction and legislation. It also created conditions of heightened human mobility and regularly endowed aliens with Roman citizenship. This chapter explores the procedural and theoretical resources developed by Roman authorities to deal with the conflicts of law that resulted from these processes. It explores principles governing choice of law, to wit, those of personality and territoriality. It also investigates procedural mechanisms that temporarily bracketed distinctions that rendered actions non-justiciable, for example, the legal fiction that assimilated aliens to citizens for the purposes of dispute resolution.