Significance: High-mobility group protein 1 (HMGB1) is an evolutionarily conserved and multifunctional protein. The biological function of HMGB1 depends on its cellular locations, binding partners, and redox states. Extracellular HMGB1 is a mediator of inflammation during infection or tissue injury. Immune cells actively release HMGB1 in response to infection, which in turn orchestrates both innate and adaptive immune responses. Recent Advances: Hyperacetylation of HMGB1 within its nuclear localization sequences mobilizes HMGB1 from the nucleus to the cytoplasm and subsequently promotes HMGB1 release. The redox states of the cysteines in positions 23, 45, and 106 determine the biological activity of the extracellular HMGB1. Critical Issues: The full picture and the detailed molecular mechanisms of how cells regulate the posttranslational modifications and the redox status of HMGB1 during immune responses or under stress not only unravel the molecular mechanisms by which cells regulate the release and the biological function of HMGB1 but may also provide novel therapeutic targets to treat inflammatory diseases. Future Directions: It is important to identify the signaling pathways that regulate the posttranslational modifications and the redox status of HMGB1 and find their roles in host immune responses and pathogenesis of diseases.