Since the publication of Directive 2001/20/EC in the Official Journal of the European Communities on May 1, 2001, the Member States have been preparing its transposition into national legislation for application no later than May 2004. The discussions have proved to relate essentially to the organisation of the Ethics Committees, which must now give a single opinion per Member State, as well as to the time limits for obtaining this opinion and the authorization for a study from the competent authority. These considerations have led to proposals of shorter time limits for products at an early stage of development or involving certain diseases. Despite harmonization of the legislation, the application of the Directive does not eliminate all national differences.