German psychiatrists proposed the extermination of mental patients before Hitler came to power. Then in Nazi Germany, organized psychiatry implemented involuntary eugenical sterilization and euthanasia, ultimately killing up to 100000 German mental patients. The six psychiatric euthanasia centers utilized medical professionals, fake death certificates, gas chambers disguised as showers, and the mass burning of corpses.Psychiatrists from the euthanasia program also participated in the first formalized murders in the concentration camps. Inmates were "diagnosed" on euthanasia forms and sent to the psychiatric euthanasia centers. These facilities later provided the training, personnel and technology for the larger extermination camps.Medical observers from the United States and Germany at the Nuremberg trials concluded that the holocaust might not have taken place without psychiatry. This paper summarizes psychiatric participation in events leading to the holocaust, and analyzes the underlying psychiatric principles that anticipated, encouraged, and paved the way for the Nazi extermination program.