The European Union (EU) has experienced a remarkable degree of change during its history: it legislates in an ever wider range of policy areas, and its institutions and decision-making processes have been reformed repeatedly. One of the most important institutional changes was the introduction of the codecision procedure in 1993, which empowered the European Parliament (EP) and transformed the EU system of governance. Following the entry into force of the Treaty of Lisbon the majority of legislation is now subject to codecision under the ordinary legislative procedure. Consequently, the operation of codecision has major implications for our understanding and analysis of the EU's legislative outputs and for studies of supranational policy-making and systemic evolution more generally. This collection takes stock of 20 years of practising and studying codecision and examines the procedure's long-term implications for the EU's institutions, politics and policies.