Patient perceptions are increasingly used to measure quality of care in a diversity of health-care delivery settings. The goals of this article are to review the current use of patient perceptions and to review what is known about the sensitivity of patient perceptions for discerning variations in care across delivery systems. This article first provides a rationale for using patient perceptions to evaluate delivery systems and reviews proposed frameworks for measuring perceptions. It then reviews illustrative studies that have used patient perceptions to compare delivery systems or that have examined associations between patient perceptions and other health-care indicators. Although the results of these studies suggest some general relations between patient perceptions and characteristics of delivery systems, findings are often inconsistent across individual studies. These inconsistencies may be related to several potential methodological limitations, including failure to account for the impact of patient mix, ceiling effects of patient responses, nonresponse bias, differences in data collection methods and timing of surveys, use of proxy respondents, and differences in survey instruments. The discussion concludes with five conceptual challenges and recommendations for further research: (1) to establish the sensitivity of patient perceptions for discerning differences across delivery systems; (2) to establish relations between alternative frameworks for measuring patient perceptions; (3) to standardize the measurement of patient perceptions; (4) to define optimal ways of presenting patient perceptions data to users; and (5) to broaden the "patient" populations in which perceptions of care have been measured.