Clinicians, researchers, and outcome stakeholders have the crucial, albeit difficult, task of quantifying when a person or group experiences important change or difference on any given outcome measure, often in response to a specific intervention. The minimal clinically important difference (MCID) provides this quantified value of change/difference for a measure. There are many methods for MCID derivation, which can result in multiple values for the same measure. Thus, it is important for potential users of MCID values to be aware of the nuances of MCID development and cautions for interpreting values. This review outlines MCID-related definitions, methods, and guidelines.