Drawing on a combination of legal analysis and fieldwork conducted with prisoners and administrators in both Denmark and the United States, this article interrogates how solitary confinement has been defined and constrained – or not – in the context of U.S., European, and international law over time. Solitary confinement has been pervasive in prisons across the world, since the first prisons opened. And solitary confinement has been surprisingly predictable over its long history, producing instances of extreme and de-humanizing brutality, inspiring ongoing debates about just how psychologically detrimental the practice is, and persistently resisting critiques based on evidence of brutality and psychological damage. The pervasiveness and predictability of solitary confinement suggests substantial limitations inherent in the newest barrage of critiques leveled by courts, scholars, international human rights bodies, and professional associations against the practice. Indeed, many critiques of solitary confinement have failed because they have promoted reformist rather than non-reformist (or abolition) agendas – a distinction articulated by Mathiesen (1974/2014).