In 2007, the certainties that had long guided financial markets evaporated. The financial and sovereign debt crisis fundamentally changed the financial system and the role of central banks. In the euro area, the European Central Bank (ECB) has gained enormous power and influence over the financial markets and national governments. In monetary policy, the ECB has introduced a wide range of new instruments to tackle the institutional and structural shortcomings of a destabilised financial sys- tem within an incomplete monetary union. This thesis makes a close assessment of these changes to analyse why and how the ECB has changed since the recent financial and sovereign debt crisis. It examine the questions: What were the ideational drivers for this change? And what were the lessons learned for the ECB from the crises? The thesis applies the economic ideational frameworks of post-Keynesianism and ordoliberalism as well as insights from neo-liberalism and Hyman Minsky on financial crisis dynamics to better understand the different forces that influence the decision-making of the ECB. The theoretical framework of epistemic communities also helps to better analyse the deeper implications behind the decisions of European central bankers and their interactions with commercial bankers. The thesis draws on 42 interviews with high-level decision-makers from central banks in Europe, commercial bankers, and government officials to investigate ideational change at the ECB and in the Eurosystem. The thesis argues that economic ideas play a major role in shaping decisions in the Eurosystem. However, during the financial crisis, the severity and the speed at which the system collapsed required policymakers to make pragmatic considerations rather than debate ideational factors. During this process, the ECB has developed its own identity that is continuously removing it from the ideational basis of ordoliberalism that shaped the founding documents.