Acquiring competences for the creation of criminal offences begs the question of legitimacy. The European criminal justice system already has such competences and many instruments define criminal offences. The legality principle is a cornerstone doctrine for legitimising criminal norms in Western legal systems. Despite already being part of the European legal order, this principle lacks a coherent theoretical and normative blueprint that shows how it should be conceived in European criminal law. This book develops such a theory for the principle of legality in European criminal law. The focus is on the legitimising and normative functions of this principle. The reader shall find a proposal for a theoretical framework that legitimises European criminal law and the accompanying normative requirements of criminal liability. Questions such as the precision of European and national implementing norms, the position of case law as a source of law and the scope of interpretative powers of European and national courts are addressed. The book uses comparative research into national systems and modern theories of criminal law to build a framework for the principle of legality. This is then instilled with special characteristics of the European legal order, such as the multi-level system of authorities and sources, pluralism and freedom of movement.