The ever incomplete single market: Differentiation and the evolving frontier of integration

Progress in market integration over the past two decades has come at the expense of growing flexibility in the laws that govern the single market (SM) as well as the way that these laws are implemented. This differentiated integration comes in four forms: soft; informal; multi-speed; and opt-out differentiation. We examine how the completion of the SM has been held back in the varied implementation of EU competition policy and variation in national corporate law, energy markets, services and taxation. These sectors and issue areas form the frontier in which the main political struggles over the future shape of the SM take place, and in which differ- entiation is most clearly manifested. The SM notion supposedly entails a concrete set of substantive policy commitments that form the basis of the ever closer union. However, increasing differentiation undermines the identification of the EUs core constitutional commitments.