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