The contributions in this volume reflect on the legal trajectory of solidarity in international and EU law and offer unique insights into the evolution and status of the principle in different fields of international and EU law. By doing so, the book also serves as a springboard for answering broader questions pertaining to what the stage of development of this principle may imply for the two legal orders and their interaction. This book, the second volume to appear in the Series, will appeal to international and EU law researchers and policy-makers alike with an interest in the nature and function of the principle of solidarity in international and EU law. Eva Kassoti is Senior researcher in EU and International Law at the T.M.C. Asser Institute in The Hague, The Netherlands and the Academic Co-ordinator of CLEER. Narin Idriz is Researcher in EU Law at the T.M.C. Asser Institute in The Hague, The Netherlands.