As I entered the ED for the start of my shift, I saw a strange look on the faces of colleagues. The C-Med radio had just cracked that we should stand by for a mass casualty event after reports of an explosion at the finish line of the Boston Marathon. What? I logged on to Twitter and immediately saw reports of severe injuries. One deep breath, and we were launched into disaster mode. We delegated a trauma code team leader and an overall ED team leader and arranged for a hospitalwide disaster response. Immediate needs were for rapid discharge of nonacute ED patients, fast transfer of planned admissions to the inpatient units, and care of noncritical patients arriving via car or on foot. Three trauma teams were rapidly assembled. Each consisted of an ED physician team leader, a pediatric surgeon, an airway doctor, bedside and recording nurses, residents and fellows, and clinical assistants. Pharmacists, radiology technicians, and administrators were also on site. We donned yellow precaution gowns, masks, and gloves, then affixed colored labels indicating our respective roles onto our gowns. I would be the ED team leader for the first injured child. I hushed the crowd of providers and asked them all to maintain order while we worked. By then, anesthesiologists, respiratory therapists, subspecialty surgeons, and others had started to arrive and were asked to wait in the hallway. We would call them as needed.