Not all member states are equally eager or able to participate in all aspects of integration, and the impact of EU policy on the member states varies across states and policy sectors. Whereas much of the literature on differentiated integration has focused primarily on formal opt-outs, this article widens the term to capture both the formal and informal arrangements for policy opt-outs as well as the differences, or discretionary aspects, associated with putting EU policy into practice. The article draws on sociological organisational theory to elaborate a broad and flexible understanding of European integration that links the literature on integration and Europeanisation, and proceeds to explore different types of European integration. The core question is therefore: what is differentiated integration?
A later version of this paper was published in Journal of European Integration, (2006) Vol. 28, No. 4, pp. 313-330