This monograph, written in 2000 by Cary Sennett, MD, PhD, is a description of the National Committee for Quality Assurance's HEDIS hypertension measure, as promulgated that year. The material also includes a discussion of the importance of hypertension control and the need to monitor providers carefully. The implementation of this measure created challenges for health plans, in terms of both performance measurement and performance improvement. The information provided herein is intended to help leaders in managed care plans prepare for those challenges, by providing a greater understanding of the problem of hypertension control, then explaining how measurement can help improvement efforts, what strategies might be helpful, and what implementation might require. The article is drawn from a summit that brought researchers together with health plan medical directors, pharmacy directors, and quality assurance directors to share knowledge and develop recommendations for others.