The role of managed-care organizations (MCOs) is to determine the existence of a benefit and how it is to be covered. Clinical guidelines and evidence-based medicine are critical parts of those decisions, and help drive much of the medical policies of MCOs. The goal is to identify and support medically appropriate and cost-effective interventions, and not cover treatments that do not have those characteristics. This will become increasingly important as more payers move to defined contribution plans, which place added responsibility for care decisions on the medical care consumer. Improving quality, creating consistency, and establishing the credibility of MCOs and their guidelines are imperative for integrating evidence-based medicine into coverage decisions. The ultimate outcome sought by MCOs and payers is improvement of the health of the populations the serve.