A rapid, safe, and successful response to a mass-shooting incident requires preparation. First responders at all levels must learn to work together to deter mass-shooting incidents from occurring in their jurisdictions and to safely respond if an attack occurs. This article provides some basic information, tools, and resources to assist in developing or updating multiagency mass-shooting preplans. The likelihood of a mass-shooting incident is low, but public safety officials must be prepared for these situations. Recent mass shootings and active shooting events in the United States and abroad have demonstrated the need to prepare local, regional, state, federal, and military resources for responding to these events.