The Patient Protection and Affordable Care Act (PPACA) as amended by the Health Care Education Reconciliation Act of 2010 makes landmark changes to health insurance markets. Individual and small-group insurance plans and markets will see the biggest changes, but PPACA also affects large employer and self-insured plans by imposing rules for benefit design and health plan practices. Over half of workers--most often those in very large firms--are covered by self-insured health plans in which employers (or employee groups) bear all or some of the risk of providing insurance coverage to a defined population of workers and their dependents. As PPACA provisions become effective, some have argued that smaller firms that offer insurance may opt to self-insure their health benefits because of new small-group market rules. Such a shift could affect risk pooling in the small-group market. This paper examines the definition and prevalence of self-insured health plans, the application of PPACA provisions to these plans, and the possible effects on the broader health insurance market, should many more employers decide to self-insure.