We live in an age of sanctions. For geopolitical reasons, powerful states and economic blocks increasingly impose unilateral measures restricting economic transactions with certain target states. These sanctions may apply to transactions between the sanctioning state and a target country but may at times also extend to transactions between third states and the target state. By imposing such 'secondary' sanctions, states aim to further isolate the target state. The extraterritorial character of secondary sanctions makes them controversial, as they impinge on third states' economic sovereignty and the latter's operators' freedom to conduct international business. This book addresses the legality of secondary sanctions from multiple legal perspectives, such as general international law, international economic law, and private law. It examines how third states and operators can legally react against secondary sanctions, e.g. via blocking legislation or litigation. It also provides economic and political perspectives on secondary sanctions. Focuses on the legality of secondary sanctions where powerful States seek to restrict trade between third states and sanctioned states. Tom Ruys is a professor of international law at Ghent University where he heads the Ghent Rolin-Jaequemyns International Law Institute. He is co-editor-in-chief of the Military Law and Law of War Review. His publications include 'Secondary Sanctions: A Weapon Out of Control' (with C. Ryngaert, British Yb IL 2020) and the Cambridge Handbook of Immunities and International Law (2019, ed. with N. Angelet). Cedric Ryngaert is a professor of international law at Utrecht University. His publications include Jurisdiction in International Law (2015, 2nd ed), Selfless Intervention: The Exercise of Jurisdiction in the Common Interest (2020), and 'Secondary Sanctions: a weapon out of control' (with T. Ruys). He chairs the Dutch Advisory Council on International Law and is the editor-in-chief of the Netherlands International Law Review. Felipe Rodríguez Silvestre is a Ph.D. researcher at Ghent University focusing on the legality of secondary sanctions under public international law. He studied law at Universidad de Chile and holds a Master of Laws degree in Advanced Studies in Public International Law from Leiden University. Felipe previously worked as a legal officer of the Ministry of Foreign Affairs of Chile in the dispute between Chile and Bolivia before the International Court of Justice, 'Obligation to negotiate access to the Pacific Ocean'.