Good Clinical Practice (GCP) is a quality assurance system dealing with all stages of clinical trials which is progressively being adopted by European countries. European GCP guidelines are in preparation and will be issued soon. However, implementation of the guidelines poses major and costly problems. The training of investigators, the proper functioning of research ethics committees, the practice of obtaining written informed consent, source data verification, and quality control with internal audit and official inspections are among the most difficult issues. The obvious benefits of GCP are the improved quality of clinical trials and of data generated by such trials, as well as mutual recognition of studies conducted abroad.