After several years of intense public debate and continuous regulatory restructuring, a new system for the control of agricultural biotechnology is currently taking shape in the European Union. Based on the ability to make a distinction between genetically modified and conventional organisms in food commodities, this system aims to detect, identify and trace throughout the production chain all bio-engineered products entering the European market. The commingling of conventional and genetically modified products, or any situation in which a single, clear-cut categorization cannot be established, is defined in practice as a form of pollution, which must be discovered, controlled and ultimately prevented. It is this policing of purity that this the focus on this dissertation research grant. The dissertation tracks the controversies surrounding testing methodologies and the emergence of the principle of traceability for genetically modified organisms (GMOs) in the EU. Given the transnational nature of the commodity flows, detection instruments and new regulatory provisions must operate beyond the strict geographical boundaries of the European Union. The dissertation explores the development of this new control system in the context of EU-US relations and compares developments on both sides of the Atlantic. Support is requested for additional fieldwork in Europe and the United States to complete the previous research. Intellectually, the dissertation draws together a number of disciplines and research interests. By analyzing the role of techno scientific instruments in the stabilization and/or disruption of transnational marketplaces and regulations, the research tries to bring Science and Technology Studies into fruitful contact with Economic Sociology and International Relations. Finally, the dissertation hopes to make a contribution to the ongoing international policy debate regarding social control of agricultural biotechnology.