An assessment instrument capable of measuring the wellbeing of the aged in a number of significant domains is described. This Philadelphia Geriatric Center Multilevel Assessment Instrument (MAI) systematically assesses behavioral competence in the domains of health, activities of daily living, cognition, time use, and social interaction and in the sectors of psychological wellbeing and perceived environmental quality. Determination of the psychometric qualities of measures of different length in each of these domains and sectors was made. The performance of 590 older people in groups composed of independent community residents, in-home services clients, and people awaiting admission to an institution was determined. The MAI is seen as useful for both research and for assessment in service-giving situations.