Approximation of EU member states' criminal law has been a major area of activities in the third pillar of the EU ever since the Treaty of Maastricht. The question of legal basis and the role of approximation have prompted a number of controversial questions; these two issues will be addressed in this paper. The perspectives opened by The Hague Program, 'Strengthening Freedom, security and justice in the European Union', and the Action Plan implementing it will be part of the analysis. It is probable that some highly necessary measures, such as the development of approximation in criminal procedure, do not require the Constitutional Treaty to enter into force. In this regard, one should underline the importance of the Hague Program and its Action Plan, both largely influenced by the spirit of the changes envisaged by the Constitutional Treaty. Regardless of the future of the Constitutional Treaty, a thorough reflection on the functions and contents of approximation is urgently needed.