In this book, the author outlines three independent bases for the existence of legal limits to the veto by UN Security Council permanent members while atrocity crimes are occurring. The provisions of the UN Charter creating the veto cannot override the UN’s ‘Purposes and Principles’, nor jus cogens (peremptory norms of international law). There are also positive obligations imposed by the Geneva and Genocide Conventions in situations of war crimes and genocide - conventions to which all permanent members are parties. The author demonstrates how vetoes and veto threats have blocked the Security Council from pursuing measures that could have prevented or alleviated atrocity crimes (genocide, crimes against humanity, war crimes) in places such as Myanmar, Darfur, Syria, and elsewhere. As the practice continues despite regular condemnation by other UN member states and repeated voluntary veto restraint initiatives, the book explores how the legality of this practice could be challenged.