The answer to the question of what health care services should be covered by a managed care plan is straightforward; the plan should cover whatever the consumer is willing to pay for. From the plan's perspective, the consumer is the payer, that is, the employer who negotiates the plan; not the individual patient whose personal preferences and interests may be quite different. Since managed care organizations contract with payers to arrange for health care services within a defined set of benefits, there is a broader question as well: Within the benefits chosen by the payer, what actually is covered? Criteria for determining "medical necessity," which managed care plans frequently use as the basis for coverage, are discussed.