Good Clinical Practice (GCP) is an international ethical and scientific quality standard for designing, conduction, recording and reporting trials that involve the participating of human subjects. Since 1997 the ICH-GCP Guideline has been a requirement for conducting clinical trials which should be used as documentation to the authorities. The 13 fundamental principles of the ICH-GCP Guideline for conducting clinical trials are described. Furthermore, the most essential responsibilities of the investigator, sponsor and sponsor-investigator are mentioned. Investigator-initiated trials do not need to be conducted according to the GCP-principles but after the implementation of the EU Directive 2001/20/EC this will be changed. Today the GCP-principles only apply to clinical research with drugs but within a few years the GCP-principles will probably be a requirement for all clinical research.