While the focus of the European Union in criminal law over the last decades has predominantly been on the implementation of the principle of mutual recognition, the EU also further developed its influence on substantive criminal law. This book addresses the issues at stake. In the first section, fundamental questions for the development of a coherent general part of European criminal law are discussed. Why does the EU need a general part of European criminal law? When a general part comes into being, how is this influenced by EU law and the law of the Member States? This discussion takes place in the light of the current legal situation and the future developments of the establishment of the European Prosecutor's Office. The goal is to pin down the position of the general part in the European criminal justice system and to show its interrelations within existing and future developments in EU law. The second section delves deeper into the 'what' question, examining the different principles and doctrines of the general part, followed by the traditional division into the principle of legality, jurisdiction, locus delicti, actus reus, and mens rea. The goal is to envisage the possible scope of these principles in European criminal law through comparative analysis of existing national law systems and EU law. In the third and final section, the focus turns on the appropriate process - the 'how' question - for the development of a general part. The goal is to bring forward possible profiles of the general part of European criminal law based on three different elements: the current (rather coincidental) influence on the general part through the Court, the interaction of the general part with the already existing special part, and the lessons learned from the development of the general part in the Model Penal Code of the US.