On Friday, November 13, 2015, Paris was subjected to a multiple terrorist attack that caused widespread carnage. Although French emergency planning, response, and resilience procedures (Plan Blanc) anticipated crisis management of a major incident, these had to be adapted to the local context of Pitié-Salpêtrière University Teaching Hospital. Health care workers had undergone Plan Blanc training and exercises and it was fortunate that such a drill had occurred on the morning of the attack. The procedures were observed to work well because this type of eventuality had been fully anticipated, and staff performance exceeded expectations owing to prior in-depth training and preparations. Staff performance was also facilitated by overwhelming staff solidarity and professionalism, ensuring the smooth running of crisis management and improving victim survival rates. Although lessons learned are ongoing, an initial debriefing of managers found organizational improvements to be made. These included improvements to the activation of Plan Blanc and how staff were alerted, bed management, emergency morgue facilities, and public relations. In conclusion, our preparations for an eventual terrorist attack on this unprecedented scale ensured a successful medical response. Even though anticipating the unthinkable is difficult, contingency plans are being made to face other possible terrorist threats including chemical or biological agents. (Disaster Med Public Health Preparedness. 2016;page 1 of 6)