The article maps the evolution of autonomous concepts in EU criminal law, particularly in the field of procedural law related to mutual recognition in criminal matters. It gives a taxonomy of autonomous concepts, ranging from the outcome of execution (notably detention), to the procedural parameters of grounds for refusal (related to the definition of trials in absentia), and concepts underpinning the very system of mutual recognition in criminal matters (the definition of judicial authority for issuing national and European arrest warrants). A further category includes the concepts linked with the effective enforcement of procedural rights within and beyond the system of mutual recognition. The article assesses the role of autonomous concepts in managing legal diversity in Europe’s area of criminal justice. Key questions include the extent to which the development of autonomous concepts in European criminal law leads to legal certainty; the extent and limits of the harmonizing effect of autonomous concepts; and the different functions of autonomous concepts in mutual recognition and in harmonization legal instruments.