Over the past two decades Morten Kelstrup’s work has been at the centre of three important intellectual innovations in political science – the study of the EU as a regional political system; European security studies; and small states in European integration. Kelstrup’s best known books (Buzan, Kelstrup, Lemaitre, and Tromer 1990; Kelstrup 1992; Wæver, Buzan, Kelstrup, and Lemaitre 1993; Branner and Kelstrup 2000; Kelstrup and Williams 2000; Kelstrup, Martinsen, and Wind 2008) stand as part of the intellectual heritage of Danish contributions to the three previous debates. For the purposes of this book, two of Kelstrup’s most important intellectual contributions come from his work on Denmark’s relations with the European Union, and his use of systems theory to understand the EU.