This volume is the result of a research project financed by the University of Milan in 2020. It addresses a very topical issue at the core of severe tensions in the current state of global politics. Indeed, the use of measures commonly referred to as ‘sanctions’ by and against international organizations (IOs) has become a recurrent feature of international relations.

The adoption of ‘restrictive measures’ by the European Union (EU) against third States within the framework of the Common Foreign and Security Policy or on the basis of other EU treaties’ provisions; the approval of institutional sanctions by the Council of Europe against Russia in 2022 or the adoption by the African Union of measures against member States not explicitly provided for in its establishing treaty; the asset freezes and travel bans levied by the United States against the Prosecutor of the International Criminal Court; the opposition raised by States through a variety of means (ranging from the withholding of financial contributions to withdrawal) as a reaction against allegedly ultra vires acts of IOs of which they are members or as a critique of the exercise of IOs’ powers: these are just some examples of the circumstances in which tensions in the relationships between States and IOs have triggered a legal and/or political reaction by the parties concerned.