Randomized controlled trials are scientific investigations considered as the gold-standard to evaluate therapeutic interventions. Randomized controlled trials may examine the safety and efficacy of new drugs and therapeutic procedures or compare the effects of two or more drugs or any other intervention. In this article, we present the essential features of these studies, as well as, factors which may bias randomized controlled trials. We also present criteria to critically appraise articles reporting randomized controlled trials, explain how to interpret the results and how to apply them to clinical practice.