The Nazi program of mass murder targeting disabled people, begun in 1939, was Aktion T4.
The program’s name came from the address of its coordinating office at Tiergartenstraße 4 in Berlin. Nazi authorities and medical professionals selected people with physical, mental, or developmental disabilities for killing, presenting the murders as a supposed measure against hereditary illness and institutional costs.
Victims were killed in gas chambers at six institutions in Germany and Austria, including Hartheim and Hadamar. The official killing phase was publicly halted in August 1941 after protests, including criticism from Bishop Clemens August von Galen, but killings continued in less centralized forms. Aktion T4 also helped develop personnel and methods later used in the Holocaust.
The term is sometimes used too narrowly for only the 1939–1941 operation; historians also discuss subsequent disability killings as part of the broader Nazi persecution of disabled people.