In rehabilitation research patient questionnaires are widely used for evaluative purposes, i. e. to measure improvements or deteriorations over time. This is only possible if the questionnaires applied appropriately reflect "true" change over time, i. e. they have to be sensitive to change. The aim of this paper is to point out the importance of the "sensitivity to change" concept for evaluative assessment tools and evaluative studies, respectively, considering quality of life research as an example. Various qualitative aspects, e. g. scaling of response options of assessment tools, are covered as well as quantitative methods, i. e. study designs and indices. Furthermore, recommendations for interpretation are given.