The lack of a clear and consistent definition for Long COVID presents challenges for policymakers, researchers, public health professionals, clinicians, support services, and patients. As such, the Administration for Strategic Preparedness and Response and the Office of the Assistant Secretary for Health asked the National Academies to assemble a committee of experts to produce a consensus definition for Long COVID. The resulting report, A Long COVID Definition: A Chronic, Systemic Disease State with Profound Consequences, presents the 2024 NASEM Long COVID Definition, developed based on findings reported in existing literature, as well as stakeholder and patient input.