Most indicators of physical health reviewed in this book and many of the psychological scales build their operational definitions of health on the concept of functioning: how far is the individual able to function normally and to carry out typical daily activities? These are effect indicators. In this view, someone is healthy if physically and mentally able to do the things she or he wishes and needs to do. The phrase “activities of daily living” epitomizes this principle. There are many discussions of the concept of functional disability; early examples include those by Gallin and Given (94) and Slater et al. (95). As Katz et al. pointed out, functional level may be used as a marker of the existence, severity, and impact of a disease even though knowledge about its cause and progression is not advanced enough to permit measurement in these terms (22, p49). Measuring functional level offers a convenient way to compare the impact of different types of disease on different populations at different times. A common approach to measuring health is therefore through the impact of disease on various aspects of function. The notion of impact is contained in the titles of several measures, such as the Arthritis Impact Measurement Scales or the Sickness Impact Profile. Stating that an index of disability will assess functioning does not, however, indicate what questions should be included. At this more detailed level, measures diverge in their conceptual basis; approaches that have been used include psychological theories such as Maslow’s hierarchy of human needs, biological models of human development (as in Katz’s ADL scale), or sociological theories such as Mechanic’s concept of illness behavior.