There were 75 firehouses in which at least one member was killed. The FDNY also lost its department chief, first deputy commissioner, one of its marshals, one of its chaplains, as well as other administrative or specialty personnel.

Operationally and geographically, the department is nominally organized into five borough commands for the five traditional boroughs of New York. Within those borough commands exist nine divisions, each headed by a deputy chief. Within each division operate four to seven battalions, led by a battalion chief and typically consisting of 180–200 firefighters and officers. Each battalion consists of four to eight companies, with a company being led by a captain. He commands three lieutenants and 16–42 firefighters. Last is the unit consisting of the members of the company on call during a given tour, consisting of a lieutenant or a captain plus a number of firefighters depending on the type of unit: three to four on an engine company, five on a ladder company (also known as a truck company), five for a rescue company, five for a squad company, four in a marine company, and six for the hazardous materials company.[6]

On September 11, the battalion chief of Battalion 1 witnessed American Airlines Flight 11 crash into the North Tower of the World Trade Center and immediately radioed a multiple alarm incident. Over the course of the next three hours, 121 engine companies, 62 ladder companies and 27 fire officers were deployed to the scene. All off-duty firefighters were recalled—the first time the FDNY had issued a total recall in over 30 years.[7] In addition to the regular fire apparatus and personnel assigned to the incident, the FDNY also deployed its only Haz-Mat unit, its mobile command center, its field communications unit, all its five rescue units, both of its high-rise units, six of its seven squad units, and one of its two tactical support units.[8]