The planning process is interpreted as a process of rational argumentation. The structure of and standards for rational arguments in any discipline are discussed and subsequently applied to the field of public planning. The principal threats to rational argumentation in planning are seen to be associated with the lack of a shared discourse structure, and with planning's borrowing much of its conceptual repertoire from other disciplines. Planning arguments, unlike arguments in most fields, must supply both empirical and normative premises for their claims. At least three types of reasoning—utilitarian, systems, and procedural—characterize the variation in form among planning arguments. The standards of rational argumentation can allow one to judge the ‘better argument’ among those based on the same type of reasoning. There are no rational means, however, for judging among competing claims of arguments based upon different types of reasoning in lieu of a public discourse on what constitutes a ‘good society’.