The primary source of scientific evidence are well designed and well executed randomised controlled clinical trials. However, the sources of scientific evidence are multifaceted. The concept of evidence based medicine is wider, encompassing the best available scientific data, the preference of patients and the professional experience of physicians. Decisions on therapies or other types of interventions are based on these three components. The authors give an overview of the evaluation of the results of randomised controlled clinical trials and present an easily applicable assessment list for practising physicians.