Depending on the definition, these difficult prisoners can make up between 0.2 percent and 5 percent of the prison population. These inmates actively seek the means to escape and continually conflict with the authorities. In the United States, the control units for these inmates appear to have developed pragmatically, without any theoretical base. The control unit in the Federal penitentiary at Marion, Ill. simply warehouses the most dangerous inmates, and the tight containment may become self-perpetuating without affecting the inmates' violent tendencies. In contrast, the Federal Correctional Institution Butner, N. Car. was established as a form of applied research for the justice model of Norval Morris. The Special Handling Units in Canada were established on a relatively firm, theoretical base with an effort at the outset to quantify the problem and develop considered responses. A series of violent incidents led to more precise operational criteria. The Scottish tradition is pragmatic, making the prison service suspicious of research and evaluation. However, Scotland's prison service could benefit from assessments like those at Butner and should develop criteria for what makes a difficult, subversive, or dangerous prisoner.