This book explains under which conditions the European Union adopts ambitious domestic climate policies. It presents a theoretical framework that combines insights from rational choice institutionalism and the constructivist literature on political rhetoric. Its main argument is that ambitious sectoral climate policies emerge in situations in which the opponents of such policies run out of socially sustainable arguments to defend their positions. The framework is applied to five case studies of EU climate policy-making concerning cars, buildings, the EU's emissions trading system (EU ETS), the promotion of renewable energy sources as well as agricultural greenhouse gas emissions.