The legitimacy of EC directives for the harmonisation of national criminal law - as provided for in recent ECJ case law and in the Lisbon Treaty - implies the advent of a European criminal law. Consequently, respecting the "nullum crimen, nulla poena sine lege" principle within the EU assumes a particular importance. This articles has two aims: first, that of examining the different features of the "nullum crimen, nulla poena sine lege" principle at European level through an in-depth analysis of the non-written general principles of EU law resulting from international Treaties and from the Constitutions of the Member States. Second, that of assessing whether the aforesaid principle is respected in the European criminal law-making process.