This chapter introduces historical institutionalism as the theoretical framework applied in the book, focusing particularly on the work of Paul Pierson. The chapter explains that historical institutionalism emphasises the importance of understanding legal and policy development as processes that unfold over time, in contrast to ‘snapshot’ accounts. The chapter further explains Pierson’s aim of using this long-term approach to provide an account of Member State constraint at EU level, by focusing on why gaps emerge in Member State control over the evolution of EU institutions and policies, why these gaps are hard to close, and how such gaps allow actors other than the Member States (such as the European Commission and the Court of Justice of the EU) to influence integration while constraining the future room for manoeuvre of all actors.

The chapter explains that this historical institutionalist approach is applied in the book to test existing accounts of EU integration in the nature conservation field. As discussed in Chapter 1, the few theoretical accounts that exist regarding the EU integration process in the nature conservation field have tended to favour the idea that the EU’s Member States lost control of EU nature policy in certain respects. The book tests the hypotheses of two leading integration theories/approaches: (1) neo-functionalism and supranational governance (the work of Sandholtz and Stone Sweet); and (2) new intergovernmentalism (the work of Bickerton, Hodson and Puetter).