Randomized controlled trial (RCT) can be said to be one of the simplest but most powerful tool of research. It is the most rigorous way of determining whether a cause-effect relation exists between treatment and outcome and for assessing the cost effectiveness of a treatment. Through the randomization, bias will be avoided by distributing the characteristics of patients that may influence outcome randomly between treatment groups so that any difference in outcome can be explained only by treatment. These characteristics might be demographic in nature in the form of age or prognostic factors such as clinical history or disease severity. For instance, menopausal status may influence outcome of treatment for breast cancer. This article is aimed at reviewing the general principles, importance of RCT and the reason for its importance in clinical research.