To enable doctors to select the best therapeutic strategy, the value of the various options needs to be determined. The best approach to the evaluation of therapeutic effectiveness and side effects is a randomised clinical trial. Traditionally, such trials have been mainly conducted to evaluate new drugs. However, other medical interventions, such as surgical and radiological procedures and lifestyle changes, also require methodologically sound evaluation. In this article, some of the basic principles of the study design of randomised clinical trials will be discussed, such as study group comparability, treatment, data collection and statistical analysis, as well as issues relating to randomisation, outcome and blinding. Attention is also paid to some of the ethical aspects.