This article addresses the problematic lack of available data on jail isolation. It discusses the potential significance of the practice of isolating jail inmates and the basis for believing that punitive isolation in jails is at least as widespread as in prisons. It also summarizes some of the information that recently has become available about the use of isolation at one notorious jail complex—Rikers Island—where the practice has been reported on and debated perhaps more than any other, and uses Rikers as both an instructive case study and cautionary tale. Finally, the article briefly reviews what is known about the significant risk of serious harm that isolated confinement is known to represent and acknowledges the need for reliable data gathering, meaningful outside monitoring, and effective oversight.