HE ´ CTOR OLA ´ SOLO A NOTE ON THE EVOLUTION OF THE PRINCIPLE OF LEGALITY IN INTERNATIONAL CRIMINAL LAW A proper understanding of the role played so far by customary international law in international criminal law, as reflected in the case law of the ad hoc tribunals, as well as the reasons why custom- ary international law has played such a relevant function in contrast to its role in national legal orders is essential to the understanding of the process of criminalisation at the international level. It is only when one realises the limits inherent in customary rules as to the development of penal norms and their inability to assure legal cer- tainty, that one can understand why the drafters of the Rome Statute have decided to leave aside to an important extent the traditional system of sources in international criminal law and have given written norms a central role in the definition of the crimes and the penalties. The requirement that the prohibited acts and the penalties must be defined by law beforehand, encapsulated in the maxims nullum crimen sine lege and nulla poena sine lege, is a manifestation of the sub- stantive dimension of the legality principle in criminal law. According to this principle, the exercise of the ius puniendi of the State – or of the international community – is subject to the principle of legal certainty as an essential component of the fundamental rights of any person. 1 * Ph.D. (Salamanca), LLM (Columbia University), Legal Officer, International Criminal Court. Senior Honorary Lecturer, Department of Criminal Law of the University of Utrecht. Former Advisor to the Spanish Delegation to the Preparatory Commission of the International Criminal Court. The views presented in this paper are those of the author alone and do not necessarily reflect those of the International Criminal Court. 1 QUINTERO OLIVARES, G., MANUAL DE DERECHO PENAL; PARTE GENERAL 68 (3rd edn., 2002). Criminal Law Forum (2007) 18:301–319 Ó Springer 2007 DOI 10.1007/s10609-007-9042-9