The minimal clinically important difference (MCID) for an instrument is a much sought after, but elusive figure. In this review we will highlight new findings in this area, including taxonomy of MCID, methods used to ascertain MCID, the perspective taken for evaluating importance, and other sources of variation for MCID values. In the end we believe the MCID will be a context-specific value rather than a fixed number. The review highlights the need to do methodological research in this area, especially concurrent comparisons between approaches, or across different patient groups. There are many faces to the MCID, it is not a simple concept, nor simple to calculate.