A super-maximum security prison, or part of a prison, in which inmates are kept under "permanent lockdown", consisting of solitary confinement in 6 by 8 foot cells for twenty-two or twenty-three hours out of the day. Prisoners in control units do not congregate for dining, religious services, or exercise, as is customary in ordinary prisons. Phone calls, visits from family and friends on the outside, and access to educational and vocational programs are severely limited.

The decision to send a prisoner to a control unit is made not by a judge at sentencing, but by prison officials, often without a hearing or due process. Many prisoners are sent to these facilities after filing lawsuits or reporting human rights abuses within prisons. They are also frequently used to house political prisoners and organizers (e.g., Leonard Peltier of the American Indian Movement) as a means of keeping them isolated from other prisoners.

Despite the extremely tight security at these facilities, and despite prisoners' limited access to media and contacts on the outside, reports of brutality, grossly inadequate medical and mental health treatment, rape by prison guards, and other human rights abuses at control units abound.

Thirty-six states currently operate control units. There is also a Federal super-max facility in Florence, Colorado.