In her paper ‘The principle of lex certa in national law and European perspectives’ Peristeridou rightly points out that the principle of legality, one of the cornerstones of the criminal law, constitutes a complicated legal doctrine and practice. While the principle of legality poses many difficulties and challenges in application on the national level, elaborating a substantive criminal law on a European level on the basis of this principle promises to be even more difficult, given the diversity of legal cultures and the multi-level decision-making.