This is the fourth in the Series of Select Proceedings of the European Society of International Law (ESIL) featuring the most important and interesting papers presented at the Fifth Biennial Conference on 'Regionalism and International Law', organised by ESIL and the University of Valencia in 2012. As usual, the best papers from that conference have been re-written, edited and drawn together by the two editors to present a perspective on what is a flourishing forum for the discussion of new ideas and scholarship on international law.