This chapter traces the history of solitary confinement practices and their effects in prisons and places of detention from the rise of the modern penitentiary in the United States and Europe during the nineteenth century and up until present day, examining methods used in different countries around the world. It discusses how various forms of isolation have been employed for very different purposes and demonstrates how the effects of solitary confinement have been discovered in different contexts during the last two centuries. Nevertheless, these effects have been forgotten or neglected at several important junctures during the history of imprisonment. Today, few doubt that solitary confinement often has powerful consequences for the mind and body of prisoners, but the degree to which lawmakers and prison administrators acknowledge this varies greatly.