Interpretation of results of clinical research should not only focus on statistical significance (P value less than 0.05) but also clinical significance. The minimal clinically significance difference (MCID) assists to answer the question with the results being clinically significant. In addition, MCID plays an important role in evidence assessment during clinical guideline development, sample size estimation for clinical trials, and clinical decision-making. This paper primarily introduces the terminology and definition of MCID and four common methods used to estimate MCID.