California is home to nearly 20% of the country’s older adults with low incomes and people with disabilities who receive health care coverage from both Medicare and Medicaid. These 1.4 million people, known as dual-eligible enrollees, are in poorer health than the general Medicare population under most measures, and are disproportionately more likely to be people of color compared with California’s total Medicare population. On average, dual-eligible enrollees constitute the highest-need, highest-cost segment of both the Medicare and Medi-Cal enrollee populations. Justice in Aging authored this primer for the California Health Care Foundation and it provides a comprehensive overview of the characteristics of the dual-eligible enrollee population in California and the current structures for administration and delivery of services, including variations within the state.