Rational approaches to policy problems of disability require some level of common agreement on both the conceptual basis and the measurement of disability. Service organizations, however, define disability in a variety of ways, depending on their, interests, orientation, objectives, or jurisdiction. The limitations of specific program criteria emphasize the need for inclusive and comparable measurements of disability in order to examine the rela tionship to and the effects of public policy. Measurement problems are re viewed, and prevalence estimates from the Social Security Survey of the Disabled are compared to those of the National Health Interview Survey and other studies in the United States. These studies, which show a con siderably higher prevalence of disability and severe disability than does the National Health Interview Survey, also indicate that the identification meth ods used are reliable and consistent. Disability and impairment estimates from five nations are compared, and the differences among the studies are reviewed in terms of their possible effects on the level of disability reported. Also examined are data on the differences between client populations and the disabled population at large. This paper points out the need for greater emphasis on the behavior processes of normalization and adaptation in dis ability. The study comparisons suggest the need for national comparative studies using common methods and criteria to provide a better basis for un derstanding and dealing with the problems of disability.