On 11 September 2001, terrorists hijacked two passenger planes and crashed them into the two towers of the World Trade Center (WTC) in New York City. These synchronized attacks were the largest act of terrorism ever committed on US soil. The impacts, fires, and subsequent collapse of the towers killed and injured thousands of people. Within minutes after the first plane crashed into the WTC, the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, initiated one of the largest public health responses in its history. Staff of the CDC provided technical assistance on several key public health issues. During the acute phase of the event, CDC personnel assisted with: (1) assessing hospital capacity; (2) establishing injury and disease surveillance activities; (3) deploying emergency coordinators/liaisons to facilitate inter-agency coordination with the affected jurisdictions; and (4) arranging rapid delivery of emergency medical supplies, therapeutics, and personal protective equipment. This incident highlighted the need for adequate planning for all potential hazards and the importance of interagency and interdepartmental coordination in preparing for and responding to public health emergencies.