EU (CJEU) and the European Court of Human Rights (ECtHR) recognising and addressing the limits to mutual trust and recognition and the role of fundamental rights in the context of mutual recognition instruments. The case law initially stemmed from asylum procedures, albeit AG Sharpston in her Opinion in Radu, raised the issue previously in the context of criminal proceedings. Gradually, as European criminal law developed, a greater number of cases raising questions surrounding cross-border criminal matters, and the fundamental rights implications arose. In April 2016, the joined case of Aranyosi and Căldăraru was brought before the CJEU for a preliminary ruling specifically addressing the concerns of a judge tasked with executing an EAW. The issue was that by executing the two arrest warrants there was a risk that the individuals concerned could be subjected to inhuman prison conditions, contrary to Art. 4 CFREU. The CJEU held that in cases where there is a real risk that the prison conditions in an issuing Member State may violate Art. 4 CFREU, the responsibility rests with the executing judge to obtain information from the issuing authority regarding the specific conditions of the prison where the individual would be sent. Moreover, it was held that courts should delay executing a warrant until such time as they are satisfied that there would be no risk of a breach of fundamental rights of the individuals concerned. This judgment, confirms that the cooperation in criminal justice matters in the EU, particularly the implementation of the Framework Decisions related to detention, cannot lead to a violation of the prohibition of ill-treatment. The Framework Decisions as a chance for the prevention of ill-treatment In addition to the Framework Decisions on the Transfer of Prisoners and the EAW, which potentially raise fundamental rights concerns stemming from poor prison conditions to lack of informed consent, there are two Framework Decisions on detention that seek to address overcrowding which can amount to, or result in, inhuman treatment. The ESO looks to reduce the overuse of pre-trial detention while the FD on Probation and Alternative Sanctions (FD PAS) relates to post sentencing measures with an aim to promote rehabilitation and address recidivism. In contrast to the enthusiasm with which Member States apply the FD EAW, FD ESO and FD PAS are much less frequently applied. The reasons for this are manifold, and include a lack of awareness of their existence amongst practitioners and the judiciary, a lack of clarity in terms of how the instruments are to be applied in practice and the bureaucracy that the process entails. How National Preventive Mechanisms can contribute to the prevention of ill-treatment in the implementation of EU law The CJEU in the case of Aranyosi and Căldăraru specifically mentions the role of national monitoring bodies, as well as European and International ones, as a source of reliable information for judges. It, however, left many questions unanswered in terms of the actual application of the test established in this case, and the practical role national monitoring mechanisms could play in the context of EU law. On foot of this judgment, the potential role of NPMs to contribute to the implementation of EU law in accordance with Art. 4 CFREU is further considered in this Study. NPMs enjoy a broad mandate in terms of access to places of detention, and as such, the Study analyses where NPMs mandate could suitably thrive in the context of mutual recognition instruments, and in particular EAWs, in providing up-to-date, first hand, reliable, independent information on prison conditions in Member States. However, also with regard to a national context, the Study suggests that engagement between NPMs 8