This book is based on a PhD thesis written between September 1997 and December 2000 at the European University Institute in Florence, under the joint supervision of Giuliano Amato and Jean-Victor Louis. The viva took place on 5 March 2001. The examining board was composed of my supervisors, Gráinne de Búrca, Koen Lenaerts and Peter Oliver. I would like to thank them for their comments, criticism and suggestions. I am especially grateful to my supervisors, who were always of great help.
