Legal uncertainty relating to measures linked to non-physical aspects under the law of the WTO is not a new phenomenon. Albeit with a focus on measures linked to processes and production methods (PPMs), it has been debated intensely in legal, economic and political circles since the early 1970s. Nevertheless, most legal questions underlying the debate have remained unsettled. The fact that this analysis builds on a decades-old debate facilitates the analysis in some respects, but also raises difficulties. The debate has given structure to the large problem-field, and it has developed concepts, some of which are used in this analysis. However, other concepts and terms developed and used in the debate have remained blurred, and their usefulness is doubtful. The intention of this chapter is to illustrate the topic and to identify the concepts and terms used in the subsequent parts of this analysis. It begins with a brief illustration of the topic detached from the legal debate, followed by a description of the landmark cases of GATT and WTO dispute settlement. The next section focuses on the debate that emerged around these disputes, the so-called ‘PPM debate’. It describes the basic concepts and terms developed for and used in this debate. Section 1.2 briefly reviews the legal status of the most important types of measures relevant to the debate in order to identify the legal core issues.