Those who accept the practice consider it necessary for prisoners who are considered dangerous to other people ('the most predatory' prisoners), those who might be capable of leading crime groups even from within, or those who are kept 'incommunicado' for purported reasons of national security. Finally, it may be used for prisoners who are at high risk of being attacked by other inmates, such as pedophiles or witnesses who are in prison themselves. This latter form of solitary confinement is sometimes referred to as protective custody.
In the US Federal Prison system, solitary confinement is known as the Special Housing Unit (SHU), pronounced [ʃü]. California's prison system also uses the abbreviation SHU, but it stands for Security Housing Units. In other states, it is known as the Special Management Unit (SMU) pronounced [smü].
Opponents of solitary confinement claim that it is a form of cruel and unusual punishment because the lack of human contact (and the sensory deprivation that often go with solitary confinement) can have a severe negative impact on a prisoner's mental state that may lead to certain mental illnesses such as depression or an existential crisis.
Interestingly, the punishment factor of solitary seems to be lessened or even eliminated in the case of some prisoners with antisocial or sociopathic personalities. Whether this is due to the flattened emotional responses present in such cases, or the extreme detachment that typify these disorders, is currently unknown.