The EU system of external relations operates through a complex web of interactions between the Union and its Member States. The book seeks to elucidate the general functioning and evolution of that system from its inception in the early 1990's to its post-Lisbon configuration, by examining the concomitant development of the Union's relations with two of its east-European partners, the Russian Federation and Ukraine. Often used as a laboratory for experimenting with new EU constitutional devices and as a testing ground for innovative institutional practices, the EU-Russia/Ukraine relations have also foreshadowed legal change in the system itself. As such, they represent invaluable case studies to expose the key features of the Union's external action.Issues covered include the law and practice of mixed agreements, embodied by the Partnership and Cooperation Agreements; the principles structuring the management of such agreements in the context of an evolving EU constitutional framework (eg conferral and differentiation of foreign policy making powers in the EU, the duties of cooperation and coherence). The analysis also exposes the significance of institutional practice in shaping the law and practice of EU external relations, as epitomised by the establishment of the European Neighbourhood Policy, and the development of EU-Russia Common Spaces.The book will be of interest to scholars, practitioners and students of the law of EU external relations, EU constitutional law, and the Union's policy towards its east-European neighbours.