Objectives: Responsiveness is one of nine measurement properties that reflect the quality of outcome measurement instruments. Methods: In this article, we explain that responsiveness is considered longitudinal validity, which refers to the degree to which an instrument is able to measure change in the construct to be measured. Results: Responsiveness should be assessed in a longitudinal design, where hypotheses are tested about (1) the expected direction and magnitude of correlations between change scores on the instrument of interest and change scores of other instruments; (2) expected differences in change scores between different subgroups (i.e. known groups); or (3) the magnitude of the change in score that is expected on the construct of interest after a treatment with known efficacy. Conclusion: Responsiveness cannot be proven, though, it is an ongoing process of testing hypotheses.