The principal effects of Brexit on health and health care will fall within the United Kingdom, and all forms of Brexit have overwhelmingly negative implications for health care and health within the UK. This article focuses on the external effects of Brexit ("Brexternalities") for health and health care. The EU is a particularly powerful institutional and legal arrangement for managing economic and political externalities in health policy as in any other policy. Equally, when a state leaves the EU, the manner of leaving will result in better or worse management of relevant externalities. Brexternalities thus involve questions about policy legitimacy and accountability. Health Brexternalities do not fall equally in all EU countries. They are felt more distinctly in the context of those elements of health policy that are most closely entwined with the UK's health policy (e.g., on the island of Ireland, certain areas of Spain, and other parts of southern Europe). Some health Brexternalities, such as in medicine safety, will be imposed on the whole population of the EU. And some health Brexternalities, such as communicable disease control, will be felt globally. Copyright © 2021 by Duke University Press.