The renewed interest in the care and treatment of chronic psychiatric patients has led to an awareness of the need for improved assessment methods for this group. This article describes the development of REHAB, a behavior rating scale for use with people with chronic psychiatric disability, which has been carefully designed with respect to content, format, and ease of use. The article also draws together the various factor-analytic, reliability, validity, and sensitivity-to-change studies that provide the basic psychometric evidence for the scale's value. REHAB is presented as an integrated package of materials with special attention paid to training of raters and simplicity of interpretation. It has been used for a variety of different assessment purposes, in both research and clinical settings.