Much attention has been directed toward the measurement of health outcomes and the quality of medical care. Some policymakers tout outcomes measurement as a promising tool for improving health care, while others question whether current quality of care assessment is valid and of practical importance. Although significant advances have been made in the ability to measure quality of care with validity, several major methodological challenges remain. We analyze the quality of quality of medical care measures, using a conceptual framework that outlines the purpose of the measures; paradigms of the quality of care, including Donabedian's structure-process-outcome model and continuous quality improvement; and other key elements, such as scope, time, unit of analysis, and perspectives. Patient preferences and organizational contexts have been underemphasized, and should be incorporated into the framework for conceptualizing quality of care. Quality of care has relativistic and dynamic aspects. Different perspectives lead to different views of what high quality care is, and quality of care is a changing concept since medical science and treatment are constantly evolving. The most appropriate quality measures depend upon whose perspective one takes and the purpose of the measures; if the measures are chosen wisely, the current state of quality of medical care measurement is adequate for both accountability and quality improvement.