This volume takes stock of twenty years of practising and studying codecision in the European Union (EU) and examines the procedure’s long-term implications for the EU’s institutions, politics and policies. The introduction of co-legislation between the Council of Ministers and the European Parliament in 1993 raised the prospect of increased parliamentary involvement in EU decision-making and promised a new era of more transparent, inclusive and accountable policy-making. This collection draws together contributions from diverse theoretical and methodological perspectives in order to analyse the extent to which codecision has delivered the expected gains and to review the unexpected effects that have followed from its introduction, such as the growing informalisation of EU decision-making. Using a combination of in-depth qualitative case studies, wider quantitative analyses, practitioners insights and a review of the procedure’s democratic legitimacy the contributions offer a holistic assessment of the effect of co-decision on the political system of the EU.This book was published as a special issue of the Journal of European Public Policy.