United States Army Air Forces Modification Centers were World War II facilities at which military aircraft underwent post-production changes in order to modify or install equipment needed for specific roles or theaters of operation. The majority of newly produced combat aircraft were channeled to the modification centers immediately after leaving the production facility, and before departing to the active theaters of war. The use of modification centers avoided disruption to the production lines to incorporate continuous improvements or other changes to the aircraft design. They were the only "set of field installations" constructed for Material Command. Modification Centers were for:

During 1942, 21 modification centers were activated, "eight being operated by commercial airlines and the remainder by aircraft manufacturers. Nineteen of the centers remained in operation at the end of the year after two other centers had been closed out":

Depots, e.g., Fairfield, Ohio (FAD), did modifications but were not modification centers.

The commercial modification centers were distinct from the USAAF Air Service Command domestic depot system, which also performed modification as an addition mission function. There were four major depots in the system which expanded to about a dozen during the war. The depots had sub-depots and auxiliary locations. There were also major overseas depot and sub-depot systems which predated addition modifications to combat aircraft.