European Union criminal law as we know it is a product of intensive legal development on both legislative and judicial levels, accompanied by significant changes in the Treaty framework. In this chapter, we will discuss the emergence and nature of harmonisation efforts with an emphasis on especially substantive criminal law. Our claim is that whereas during the Amsterdam era, harmonisation of substantive criminal law was mainly regarded a competence issue, since entering of the Lisbon Treaty together with strengthening of the role of the fundamental rights and human rights in the law of the European Union, the debates concerning European criminal law take place mainly on the level of adjudication. Harmonisation relates not only to the cornerstone principle of mutual recognition but has a link as well to the debates on increased supranationalism as expressed by ‘Taricco saga’ and the establishment of the European Public Prosecutor’s Office.