Nowadays the Randomised Controlled Trial (RCT) is seen as the gold standard for estimating the effectiveness of an observed intervention, achieving the highest hierarchy of evidence of primary research settings. Its study design basically includes two groups of patients, an intervention group and a control group; patients are randomly allocated to these two groups. After intervention or control intervention took place, predefined outcomes are quantified and compared in the two groups. The study design aims at eliminating all confounding and distorting factors (Bias and Confounder), so that different outcomes between the groups can be only explained by the intervention. There is a broad variation of quality of published RCTs. The reliability of results and extent to which findings provide a correct basis for generalisation to other circumstances needs to be validated. As part of a methods series of the Wiener Medizinische Wochenschrift this paper will discuss principles of study design, critical appraisal and limitations of RCTs.