Medicare is an insurance program. The reason we have health insurance at all is not that health care is expensive, but rather that there is great uncertainty about who will need very expensive and potentially lifesaving care and when they will need it. Medicare should give beneficiaries not just access to medical care, but also protection from the risk of catastrophic spending. At the same time, Medicare — like any good insurance — should not cover so much care so generously that beneficiaries end up consuming too much care of questionable value and driving up costs for everyone. Thus, setting cost sharing for Medicare beneficiaries is a balancing act: too little cost sharing means patients have no incentive to spend Medicare dollars wisely; too much means Medicare fails to perform its insurance function.