Significance: Stroke is the leading cause of morbidity and mortality worldwide. Inflammatory cascades have a major impact on outcome and regeneration after ischemic stroke. High-mobility group box 1 (HMGB1) has come into the focus of experimental and clinical stroke research because it is released from necrotic brain tissue and its differential redox forms attract and activate immune cells after ischemic brain injury. HMGB1 is a potent inducer of inflammatory cascades, and thereby, secondary deterioration of neurological outcome. Recent Advances: The role of HMGB1 in sterile inflammation is well established. Emerging evidence suggests that HMGB1 modulates neuroinflammation after experimental brain ischemia and that it may be a useful prognostic biomarker for stroke patients. Critical Issues: HMGB1 is instantly released from necrotic cells in the ischemic core and activates an early inflammatory response. In addition, brain-released HMGB1 can be redox modified in the circulation and activate peripheral immune cells. HMGB1 concentrations correlate with disease severity and outcome after brain injury. This is the first review depicting the crucial role of HMGB1 in the initiation and perpetuation of secondary immune alterations after experimental and clinical stroke. Future Directions: HMGB1-dependent signaling pathways are on the verge and have the potential to become a central topic in experimental stroke research. Current and upcoming projects in this field will be paving the way for future translational approaches targeting the center of poststroke inflammation to improve stroke recovery and long-term outcome. Antioxid. Redox Signal. 24, 635–651.