The United States is in the midst of a bold experiment. Medicare is moving rapidly away from traditional fee-for-service payment. Various alternative payment models (APMs) intended to improve quality and reduce costs are being implemented or tested. The stakes are high. If an evaluation finds that a new model meets the statutory requirements for expansion, the US Secretary of Health and Human Services is authorized to expand the models across the country. Whether these new programs work and how effectively they are evaluated should matter to physicians, patients, and taxpayers.