Insurance claims records, which document many aspects of the process and outcome of medical care, are a practical and unobtrusive source of data for monitoring the quality of care provided to enrollees--a purpose for which they are rarely used. Data for these potential indicators could be drawn from claims or other administrative data systems. The authors established categories of care that could be used to develop claims-based indicators and compiled an annotated list of broad indicators for assessing the quality of care. The compilation of indicators is preceded by a discussion of some of the issues and challenges facing those who use and interpret claims-based indicators of quality.