This chapter explores the role of the European court system in establishing the foundations of European criminal law and influencing its subsequent development. It opens with an historical overview of the emergence of the various elements within the judicial system focusing on the Court of Justice of the EU (CJEU) and the European Court of Human Rights. Particular emphasis is placed in the discussion on the contribution of the CJEU to the field of EU criminal law as it evolved during the Union’s formative years. Through its case law, the Court acted at various times as ‘the motor of European integration’ and the arbitrator of disputes over EU competence and the primacy of EU law. The chapter then moves to the contemporary setting of judicial engagement with the principle of mutual recognition by exploring the implications of certain judgments on the European Arrest Warrant. Consideration is given in this discussion to the increasing and progressive emphasis within the jurisprudence on the essential role of the European court system in ensuring due process and safeguarding fundamental rights.