This report addresses how the term medical necessity is defined in private health insurance coverage decisions. It summarizes a review of the literature, an extensive review of legal cases that challenge insurer decisions, materials prepared by the insurance industry, consultation with experts in the field, a review of investigations conducted by State departments of insurance and attorneys general, and interviews with health care executives regarding the decision making process itself. The report does not explore factors that can affect access to care that might be considered clinically necessary by treating professionals or the effects of medical necessity decisions on therapeutic outcomes.