The Older Americans Resources and Services (OARS) methodology was designed to assess functional capacity in five dimensions (social resources, economic resources, mental health, physical health, and activities of daily living) and to measure use of and need for 24 types of generic services. The OARS questionnaire now has been used in more than 150 research and practice settings. Recent refinements in the OARS methodology include new information about validity and reliability, computerized summary ratings for the five dimensions of functional status, and eleven scales that measure specific aspects of functioning within the five dimensions. This paper describes the conceptual foundation, appropriate uses, psychometric properties, and recent refinements in the OARS methodology. Empirical data from a geriatric clinic population are used for purposes of illustration. The paper presents an assessment of the strengths and limitations of the OARS methodology based on more than a decade of experience. Particular attention is paid to the degree to which the measures of functional status can be related to health service use measures to inform geriatric research and practice.