Studies of response-shift phenomena in quality-of-life (QOL) research have grown steadily in the more than two decades of research. As a field, we have been calling a lot of different approaches “response shift” over the years, but the only approach that fully embodies the foundational (Sprangers and Schwartz, Soc Sci Med 48(11):1507–1515, 1999) theoretical model is appraisal. According to the definition proposed in this model, response shift is about individual cognitive change. This paper presents the argument that all response-shift research models need to be grounded in an understanding of cognitive appraisal. We present a reasoned argument for why appraisal is fundamental to QOL response-shift research. We describe current measurement and analytic methods for working with appraisal, and how these methods can be integrated into the current response-shift statistical ‘tool box.’ We propose future research directions on theory, methods, and cross-calibration of group- and individual-level methods. There are currently three tools available in multiple languages for assessing QOL appraisal processes. They have been tested and used to assess response shift in empirical studies of ~ 7000 people with chronic illness. The study findings illustrate how appraisal theory and methods can facilitate methodological investigations of and to enhance other response-shift detection methods. Future research directions are proposed to enrich QOL theory, response-shift methods, and interpretation of QOL change over time. Appraisal theory and methods are the closest approximation to a response-shift ‘gold standard.’ They provide the foundation for understanding response shift and point to a unified theory of QOL.