Solitary confinement involves isolation of prisoners for up to 23 hours a day, a condition that often lasts for several years and, in some cases, for 20 years or more. Estimates of the number of prisoners held in such conditions in the United States are as high as 80,000. By comparison, less than 50 inmates are thought to be held in solitary confinement in Great Britain. One such prison in the U.S. is the “super-max” California facility, Pelican Bay State Prison, which houses more than 1,000 prisoners in 8 by10 foot concrete cells with no windows and meal delivery though a slot in the door. Although claims have been made that solitary confinement reduces crime and recidivism, the evidence for this is scant. Moreover, some clinical studies suggest that such conditions can have severe psychiatric and even neurologic consequences for the inmates. This possibility is further supported by studies of prisoners of war, sensory deprivation, and environmental enrichment, and research with laboratory animals in which behavioral assessments have been supplemented with neurobiological analyses. This symposium explores the use of solitary confinement in prison systems, discusses its possible impact, and raises questions about whether it constitutes cruel and unusual punishment according to the U.S. Constitution, the European Convention on Human Rights, and other national and international conventions and statements.