Purpose. Measurement reactivity is defined as being present where measurement results in changes in the people being measured. The main aim of this review is to provide an overview of the current state of knowledge concerning the extent and nature of psychological measurement affecting people who complete the measures. Other aims are to describe how this may affect conclusions drawn in health psychology research and to outline where more research is needed. Methods. Narrative review. Results. Several studies, using a variety of methods, have found measurement procedures to alter subsequent cognition, emotion, and behaviour. In many instances, the effects obtained were of up to medium size. However, the extent to which such studies are representative is not clear: do other studies which find no reactive effects of measurement not exist or do they exist but are not reported? Conclusions. Although measurement reactivity can yield medium-sized effects, our understanding of this phenomenon is still rudimentary. We do not know the precise circumstances that are likely to result in measurement reactivity: we cannot predict when problems are more likely to arise. There is a particular absence of studies of the mechanisms by which measurement reactivity arises. There is a need for a systematic review of this literature, which should aim to quantify the extent of measurement reactivity effects and to provide a firmer evidence base for theorizing about the sources of reactivity. © 2010 The British Psychological Society.