J. Zijlstra and H.J. Witteveen: From Thought to Action Economic theory in politics and policy making (1945-1982) The two Dutch economists Jelle Zijlstra (1918-2001) and Hendrikus Johannes Witteveen (1921-2019) were academics, politicians and financial/monetary policymakers. They belonged to a small group of economists who shaped the new Dutch policy institutions and economic policy after World War II. This dissertation examines how differences in academic thinking between them drove the differences in their economic policies and draws conclusions on how academic insights can improve policymaking. In part I of this dissertation, the theoretical ideas and methodological approach that captured Zijlstra en Witteveen as students and PhD candidates are analysed and positioned within Dutch economic thinking. Part II shows how, for nearly four decades, Zijlstra and Witteveen clashed as academics, politicians and monetary policymakers. Driven by their scientific convictions they came to very different positions on fiscal policy, monetary policy and the way empirical evidence in policy making should be used.Witteveen wanted to capture the economy quantitatively and test economic theory empirically. He looked through the macroeconomic lens of aggregated analysis and used statistics not only to describe economic developments, but also to explain and predict them. Zijlstra assessed the economy through a microeconomic lens. In his view the economy was too complex to understand in full. He described the economy mostly qualitatively and in verbal terms. The policies of Witteveen and Zijlstra differed but often not because of party political differences or ideology. In many cases it were the differences in their academic thinking about economic equilibrium, the interaction between the monetary and real economy or the use of empirical models that drove the differences in their economic policies. Witteveen's tax based business cycle policy, known as the wiebeltax and Zijlstra's budget norm, known as the Zijlstra-norm are ...