Background: ICH (International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceutical for Human Use) provides guidelines on the implementation of clinical trials. All study participants are obliged to follow these guidelines in line with "Good Clinical Practice". Methods: The main features of a clinical study include the following items: Background and general aims, specific objectives, patient selection criteria, treatment schedules, methods of patient evaluation, trial design, registration and randomization of patients, patient consent, required size of study, monitoring of trial progress, forms and data handling, protocol deviations, plans for statistical analysis and administrative responsibilities. Results: All items mentioned above should already be discussed in the planning stage of a clinical trial and addressed in the study protocol. The study protocol provides a guideline for any person involved in the trial. Conclusions: For the success of a clinical trial, it is especially important to have a clear and exact definition of the study hypotheses and to choose primary and secondary endpoints very carefully. © Springer-Verlag 2006.