From the medical or disease perspective, patients' functioning, disability and health are seen primarily as the consequences or the impact of a disease or condition. In this perspective, self-administered health status instruments are used primarily to evaluate the effects of drug treatments or surgical interventions. The interpretation of these measures is generally based on scales and scores and not on individual items. Currently used instruments are reviewed and an algorithm for the selection of instruments is provided. In the rehabilitation or disability perspective, patients' functioning and health is associated with and not merely a consequence of, a condition or disease. The basis for the understanding of functioning, disability and health--in association with the condition but also the personal and contextual factors--is WHO's International Classification of Functioning, Disability and Health or ICF. This chapter illustrates the use of the ICF framework for structuring patients' problems, and the use of condition-specific ICF-Core-Sets to check for problems typically encountered in patients with a given condition.