What was the role of economists and economics in the making of the European Union? This book sheds light on the activities of a particular type of economic experts: the macroeconomic modellers of the Directorate General for Economic and Financial Affairs (DG II), who built multi-country macroeconomic models of the European Economic Community. These models aim at studying the macroeconomic interactions between national economies and at providing insights about the effects of coordinating (or not) national economic policies. The book documents the origins, challenges, and progress made by DG II experts through building these models. This book provides insight into the influence of multi-country models on modern economics. It will be relevant to students and researchers interested in the history of economic thought and the development of macroeconomic policy.