We report on significant Centers for Medicare and Medicaid Services (CMS) regulatory changes to beneficiary assignment to Accountable Care Organizations (ACOs). Changes include prospective rather than retrospective assignment, will increase the likelihood beneficiaries receiving services only from certain non-primary care specialists are appropriately assigned. The regulations also improve the ability for Federally Qualified Health Centers (FQHCs) and Rural Health Clinics (RHCs) to participate in ACOs.