Unemployment insurance (UI) provides temporary income support to workers who have lost their jobs and are seeking reemployment. This chapter reviews the origins of the federal-state UI system in the United States and outlines its principles and goals. It also describes the conditions for benefit eligibility, the benefits themselves, and their financing. The UI system is complex and includes many interested parties, including employers, worker advocates, state UI administrators, and the federal government. These parties’ differing views have led to controversies over benefit eligibility, adequacy, and whether the states or federal government should bear primary responsibility for UI. The Great Recession caused most states’ UI trust funds to become insolvent and led to renewed debate over the structure and financing of the system.