Two million head injuries occur each year (175 to 200 per 100,000 population) in the US, with the annual $80 billion dollar cost. Dr. Carney, Dr. Ghajar, and colleagues sought to identify and describe the most prevalent signs, symptoms, and deficits following head trauma. Comprehensive review of 5437 abstracts, 1362 full-text papers, and 231 selected studies resulted only in 26 studies meeting rigorous Guidelines criteria. Four concussion "prevalence indicators" were identified within 2 days of injury: 1) Observed disorientation or confusion immediately after injury; 2) Impaired balance within 1 day; 3) Slower reaction time; 4) Impaired verbal learning and memory. Two well-designed studies support greater vulnerability for individuals with multiple concussions. This work forms the foundation for future diagnostic and therapeutic guidelines for concussion.