Preventing mass shootings in the United States, particularly those occurring in school settings, is an important priority for families, government leaders and officials, public safety agencies, mental health professionals, educators, and local communities. What does the evidence say about how to detect, prevent, and respond to these tragic events? This article includes the five findings learned through National Institute of Justice (NIJ)-sponsored research: (1) Most people who commit a mass shooting are in crisis leading up to it and are likely to leak their plans to others, presenting opportunities for intervention; (2) Everyone can help prevent school mass shootings; (3) Threat assessment is a promising prevention strategy to assess and respond to mass shooting threats, as well as other threats of violence by students; (4) Individuals who commit a school shooting are most likely to obtain a weapon by theft from a family member, indicating a need for more secure firearm storage practices; and (5) The overwhelming majority of individuals who commit K-12 mass shootings struggle with various aspects of mental well-being.