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.
KEY WORDSBicameralism; codecision; legitimacy; political system. The last two decades have witnessed a lively debate about the origins, functioning and impact of codecision (for a recent literature review, see Rasmussen [2012]). The existing literature can be divided into studies that primarily focus on (a) the practical functioning and institutional development of the procedure (e.g., Farrell and Héritier 2003; Hix 2002; Roederer-Rynning and Schimmelfennig 2012), (b) the consequences of codecision for the inter-institutional balance of power between the institutions (e.g., Burns 2005; Kreppel 2002; Tsebelis and Garrett 2000), (c) the intra-institutional repercussions of the EU's legislative practice, for instance on the distribution of power within the institutions (e.g., Farrell and Héritier 2004; Rasmussen 2011), and (d) its normative implications (e.g., Shackleton and Raunio 2003; Stie 2013).Even though the literature covers many aspects of the EU's legislative practice, most studies have analysed the impact of codecision on the distribution of power between the EU's institutions; other developments and repercussions of the procedure have received considerably less attention. Moreover, existing studies rarely combine different theoretical and methodological approaches. In particular, no attempt has been made to assess systematically how the introduction of codecision has affected the functioning of the EU's political system as a whole. Yet there are good grounds for doing so, as the empowerment of the EP under codecision has transformed relations between the EU's constituent institutions and has wider implications for the legitimacy of policy outputs and the wider political system. This collection therefore brings together scholars and practitioners who represent different approaches to codecision, in order to provide a holistic picture of the effects of co-legislation upon the EU's political system. Specifically, it seeks to address the following questions: