The chapter outlines the distinct features and challenges of the legal regimes for the cooperation by states with international criminal jurisdictions. State compliance with the tribunals’ orders or requests for cooperation is key to their effective functioning absent direct law-enforcement and given the necessary reliance upon national authorities for executing arrests, obtaining evidence, enabling the collection of evidence, carrying out coercive measures, enforcing sentences, and so on. While being in theory more stringent than the ‘horizontal’ schemes for inter-state cooperation in criminal matters, the cooperation regime remains the Achilles heel of international criminal justice in the lack of failproof mechanisms for compelling compliance. The chapter sets out the scope, parameters, and addresses of the obligations to cooperate with UN ad hoc tribunals and the International Criminal Court; the extent to which such obligations can be allowed to prevail over conflicting duties; and the ability of parties in the proceedings to seek cooperation from the respective duty-bearers. It also discusses issues arising in connection with the cooperation for the arrest and surrender of suspects and related to other forms of assistance to international criminal jurisdictions, as well as the challenges of enforcing cooperation obligations and addressing non-compliance.

The chapter explains the principles of inter-state cooperation in criminal matters in connection with domestic proceedings. It sets out the international legal framework governing criminal law cooperation among states, including multilateral and bilateral agreements of general application and treaties specifically concerning international crimes, most notably the 2023 Ljubljana–The Hague Convention on International Cooperation in the Investigation and Prosecution of the Crime of Genocide, Crimes Against Humanity and War Crimes. The chapter sets out the differences between the traditional forms of assistance in criminal matters, which are associated with a higher degree of formality and subject to broader grounds for refusal and, on the other hand, the more advanced forms of cooperation (exemplified by mutual recognition in the European Union context) prioritizing effective enforcement while recognizing limited grounds to refuse requests. The chapter reviews the prerogatives of the requested states to refuse under these models with reference to double criminality, specialty, statutory limitations, ne bis in idem, and human rights concerns. It then takes a closer look at the traditional forms of cooperation and the issues which arise in that context, focusing on extradition, other mutual legal assistance, transfer of criminal proceedings, and enforcement of foreign penalties.

This chapter begins by addressing classic extradition, its historical roots and the key moments in its evolution, notably its permeation by fundamental rights after World War II, which has caused it to shift from a bilateral political arrangement to a ‘triangular’ (Eser) legal procedure where the individual concerned plays an active role. It assesses the main grounds for refusal – such as nationality, political offences and dual criminality –, and the variation into which they have developed within the European Union. The chapter then delves into the European Arrest Warrant, where proceedings have been fully judicialised and grounds for refusal considerably narrowed. The authors examine the question of whether (and to which extent) the European Arrest Warrant and the underlying principle of mutual recognition have brought about a radical change of paradigm, especially in the light of the (welcomed) ruling of the Court of Justice of the European Union in Aranyosi and Căldăraru. The conclusion anticipates the challenges that lie ahead and underscores the key role of the Court of Justice for the preservation of the whole European arrest warrant system.

The nature and specific operation of the system of judicial recognition and enforcement within the Union is based on mutual trust in the judicial systems of other Member States. Within this framework, recognition and enforcement operate with formalised quasi-automaticity. This chapter outlines the major phases through which that system developed to reach its current shape and examines case law developments regarding the substantive and formal requirements that, albeit minimally, condition the free circulation of judgments within the Union. It concludes with remarks inspired by recent developments concerning judicial independence as a fundamental pre-requisite for the mutual trust upon which the functioning of judicial cooperation within the Union relies.