A Certificate of Medical Necessity is a document required by Centers for Medicare and Medicaid Services to substantiate the medical necessity of an item (e.g., a durable medical equipment) or service furnished to a Medicare beneficiary. Medical necessity is a legal doctrine, related to activities which may be justified as reasonable, necessary, and/or appropriate, based on evidence-based clinical standards of care. By statute, Medicare may only pay for items and services that are "reasonable and necessary for the diagnosis or treatment of illness or injury or to improve the functioning of a malformed body member", unless there is another statutory authorization for payment. - adapted from wikipedia.