Medicare Advantage (private insurance for Medicare beneficiaries) presents major risks for insurers. First, the Centers for Medicare & Med- icaid Services annually compiles performance data and assigns every plan from one to five stars. These star ratings have two effects: Higher- rated plans get higher payments and more customers. Because star rat- ings depend on the customer and clinical service offered by providers, who are usually independent contractors, profitability depends on fac- tors that insurers control imperfectly. Second, profitability in Medicare Advantage depends on Medicare spending levels, and no one can really forecast how payment innovations will change Medicare spending. For example, Medicare’s bundled payment for joint replacement reduced costs by 20 percent in some markets. Changes that large could matter. Third, no one knows what will happen to Medi- care Advantage enrollment if Medicare’s benefits or payment systems change. Most insurers have profited from Medicare Advantage, benefit- ing from the enrollment of younger retirees, more efficient use of care, and more favorable contracts with providers. But some insurers, such as Catholic Health Initiatives, have posted large losses