Federal regulations establish special enrollment periods-times outside of open enrollment periods-during which people may enroll in or change their health insurance plans offered through the federal and state-based exchanges, or Marketplaces. To be eligible, a person must experience a shift in income or another "qualifying life event," such as a change in marital status or the number of dependents, or the loss of minimum essential health coverage. We produced an upper-bound estimate that 3.7 million nonelderly adults with coverage through a federal or state Marketplace could have experienced a qualifying life event and become eligible for a special enrollment period because of income shifts. In addition, more than 8.4 million nonelderly adults who did not have Marketplace coverage-three-quarters of whom had no insurance- became eligible for a special enrollment period as a result of other qualifying life events. Many if not most of these people may be unaware of their eligibility. In states that did not expand Medicaid eligibility, we estimated that 1.9 million people experienced income shifts outside of the open enrollment period that would make them eligible for Marketplace subsidies. However, because they were uninsured or had nongroup coverage (instead of Medicaid) during the most recent open enrollment period, they had to wait until the next period to enroll in a Marketplace plan.