This study examines the stability of legislative decisions in the European Union before and after enlargement and after the possible entry into force of the Lisbon Treaty. Stability of decisions is analysed both in terms of a risk of decision cycles and in terms of the frequency of gridlock situations. The risk of such ‘chaotic' decisions depends on three parameters, the applied decision rules, the dimensionality of the conflict space and the heterogeneity of actor preferences. Dietrich Drüner uses the concepts of the core and the winset, which stem from spatial voting and veto players theory and which incorporate the three parameters. Testing these concepts against a data set about 70 legislative proposals, Dietrich Drüner generates empirical findings about the impacts of enlargement and institutional reform on the EU's decision-making processes. This book is intended for scholars of European integration and politics as well as policy makers at both the European and the national level.