The duration and conditions of penal confinement (i.e., segregation) in use by our prisons, and regularly upheld by the courts, are so extreme and so harmful that ultimately such confinement should be prohibited as a matter of law and policy. Correctional officials, and the courts, tend to conflate the need to insulate some inmates from each other with the use of a 23/7 regimen of segregation, devoid of social interaction. Inmates suffering with mental illness or who are at risk from such confinement and juveniles are the exceptions, and they have had some success in the courts. This article reviews the relevant history of penal isolation, Supreme Court decisions and other case law, and the evidence of harm caused by extreme penal isolation. It is proposed that the law relating to the acceptable uses of mechanical restraints serve as an analogy for the basic reform in the use of penal isolation.