This thesis analyzes major political crises and their outcomes throughout the entire process of European integration. Its objective is to provide a theoretically informed and empirically grounded explanation to the main research question: why has there been such variation in crisis outcomes? It finds that the nature of the crisis origin, the level of member-state interdependence, and the presence or absence of French-German leadership determine a crisis outcome. Overall, this thesis compiles a systematic and comparative analysis of major crises and crisis outcomes that so far has been missing in the scholarly literature. The findings show that crises vary greatly in their outcomes and the impact they have on European integration and the polity of what today is the European Union (EU). Other than conventional wisdom might have it, crises have been an inherent part of European integration since the beginning. Moreover, only a few crises led to 'more' European integration, notably to an increase in the power of supranational institutions and procedures, while the majority did not. This thesis follows a political understanding of crises and assesses change to the system, that is, to European integration and the EU polity. It scrutinizes why and how crises of European integration came about; how they were dealt with and resolved; and in which way they affected the system. This thesis introduces and tests four possible outcome categories as the result of a crisis, namely 'transformation', 'adaptation', 'stagnation', and 'regression'. Transformation implies a fundamental restructuring of the system. Adaptation means a confirmation and some smaller adjustments to the system. Stagnation implies that no new system replaces the former system. Finally, regression means a reduction in the functions that the system fulfills. These outcome categories capture and reflect changes to the EU system in the aftermath and as a result of a crisis. They represent shifts in political competences between the European and the national level, in member-state relations, and in the overall dynamics of the integration process. As such, they go beyond the usual, oftentimes strictly institutionalist or legalist understanding of crisis and crisis outcomes that are expressed in terms of 'more' or 'less' integration. Moreover, this thesis argues that three explanatory factors reflecting the three temporal stages of a crisis (origin, management, and resolution), determine a crisis outcome. First, the type of origin (exogenous or endogenous) has important implications for the further course of a crisis and the scale of change it provokes to the EU system. Second, the level of interdependence between the EU member states, that a crisis reveals or reinforces, makes a common European response more or less likely. Third, the presence or absence of political leadership on the part of France and Germany, as the EU's two largest and potentially most influential member states, is crucial for the crisis resolution. Jointly, interdependence and French-German leadership decide on the convergence or divergence of member states' positions and stances on the crisis and the prospects of a common European crisis response. To test these propositions, this thesis analyzes eight major crises of European integration and their outcomes. These crises all threatened key features and principles of European integration and ultimately put the EU as a polity at risk. They are thus considered quasi-constitutional crises. The eight crises to be examined are: the crisis of the European Defence Community; the empty chair crisis; the (1973) oil crisis; the (British) budgetary rebate crisis; the end of the Cold War (and German unification) crisis; the Constitutional Treaty crisis; the Euro crisis; and the migration crisis. Strikingly, these crises show a great variation in their outcomes, with two crises each representing one of the four possible outcome categories. In methodological terms, this thesis conducts both within-case analysis and cross-case comparisons. For each crisis, it puts its theoretical framework and propositions to a congruence test with the empirical record, combined with a careful tracing of important events. This thesis builds on several and diverse primary sources, including archival material, European Council Conclusions, national policy documents, the memoires of leading policymakers and civil servants, and press reports. These, it triangulates and complements with specialized secondary literature available on the individual crises. This thesis contributes to the scholarly literature on European integration history and theory, EU crisis politics, and France-Germany in Europe. Theoretically, it explains variation in crisis outcomes with a small number of explanatory factors. Empirically, it shows that, despite the variation, some crises follow similar patterns. This is because of the values that the theorized explanatory factors take. The individual and comparative findings from the various case studies provide a more nuanced picture of the relationship between crises and European integration than most other scholarly accounts.