Prosecution of perpetrators of serious crimes of international concern before the permanent International Criminal Court, set up in 2002, has been few and far between. Hope thus rests with the implementation of the international legal obligation for States to either extradite such perpetrators to another State able and willing to prosecute them or prosecute the perpetrators themselves or surrender them to be prosecuted by a competent international court. This book is written by the Chairman of the UN International Law Commission's Working Group on the Obligation to Extradite or Prosecute (aut dedere aut judicare). The Commission submitted its Final Report on that topic to the UN General Assembly in 2014, leaving unanswered numerous important issues such as the customary international law status of the said obligation, immunities of State officials from foreign criminal jurisdiction, the exercise of universal jurisdiction, and competing rules of international law regarding the surrender of persons to a competent international court. This book is an authoritative guide to, as well as the unique drafting history of, the International Law Commission's Final Report. In addition, it provides a comprehensive analysis of the subject, including issues not settled by the Commission and proposing practical solutions to the daunting challenges facing international efforts to bring to account perpetrators of serious atrocities that shock humankind. It will be useful to States, law enforcement officers, prosecutors, judges, international lawyers, students of international law, and the civil society entrusted with human rights protection.