This article is part of a collaborative methodological series of narrative reviews on biostatistics and clinical epidemiology. This review aims to present basic concepts about the minimal clinically important difference and its use in the field of clinical research and evidence synthesis. The minimal clinically important difference is defined as the smallest difference in score in any domain or outcome of interest that patients can perceive as beneficial. It is a useful concept in several aspects since it links the magnitude of change with treatment decisions in clinical practice and emphasizes the primacy of the patients perception, affected by endless variables such as time, place, and current state of health, all of which can cause significant variability in results.