Segregated housing, commonly known as solitary confinement, is increasingly being recognized in the United States as a human rights issue. While the precise number of people held in segregated housing on any given day is not known with any certainty, estimates run to more than 80,000 in state and federal prisons—which is surely an undercount as these do not include people held in solitary confinement in jails, military facilities, immigration detention centers, or juvenile justice facilities. Evidence mounts that the practice produces many unwanted and harmful outcomes—for the mental and physical health of those placed in isolation, for the public safety of the communities to which most will return, and for the corrections budgets of jurisdictions that rely on it for facility safety. Yet solitary confinement remains a mainstay of prison management and control in the U.S. largely because many policymakers, corrections officials, and members of the general public still subscribe to some or all of the common misconceptions and misguided justifications addressed in this report. This publication is the first in a series on solitary confinement, its use and misuse, and ways to safely reduce it in our nation’s correctional facilities made possible in part by the Robert W. Wilson Charitable Trust.