In response to a request by the Health Care Financing Administration (HCFA), the Institute of Medicine proposed a study to examine definitions of serious or complex medical conditions and related issues. A seven-member committee was appointed to address these issues. Throughout the course of this study, the committee has been aware of the fact that the topic addressed by this report concerns one of the most critical issues confronting HCFA, health care plans and providers, and patients today. The Medicare+Choice regulations focus on the most vulnerable populations in need of medical care and other services-those with serious or complex medical conditions. Caring for these highly vulnerable populations poses a number of challenges. The committee believes, however, that the current state of clinical and research literature does not adequately address all of the challenges and issues relevant to the identification and care of these patients.