Since 2006, the European Union (EU) has increasingly made use of bilateral trade relations, and thus departed from its earlier commitment to interregionalism and multilateralism. Two examples for this are the EU's shift from interregional to bilateral relations with the Mercado Común del Sur (MERCOSUR) and its regional power, Brazil, and with the Association of Southeast Asian Nations (ASEAN) and its economically most important member Singapore. This turn to bilateralism is particularly puzzling in the cases of MERCOSUR and ASEAN because of the EU's long-lasting relationship with these regional organizations and because of the EU's financial support for their regional integration. Drawing on realist theorizing, this turn to bilateralism can best be explained by the EU's motivation to secure its economic and regulatory power in South America and Southeast Asia, and by the regions' varying levels of cohesion. Factors rooted in the international system rather than inner-institutional characteristics have shaped the EU's trade policy which calls the explanatory power of liberal approaches into question. Testing an alternative theoretical model coined commercial realism against commercial liberalism and the principal-agent framework, the analysis sets out the scope condition of theorizing and analyzing EU external economic policy from a realist perspective. Employing original data from 165 media press articles, 48 standardized interviews from a survey by Dür and De Bièvre (2007), 44 standardized interviews from an original survey with interest groups enrolled in the Civil Society Dialogue, 66 consultation sheets of the European Commission's consultation on EU future trade policy, and 46 elite interviews, this thesis analyzes the EU's recent switch in approach in a comparative fashion. A combination of primary and secondary cases, triangulation of data and methods, and a combination of research strategies, including rigorous process-tracing, maximizes the research design's external and internal validity.