One of the most serious human rights violations today is occurring throughout the US. In US jails and prisons, individuals are held in solitary confinement for weeks, months and even years. Solitary confinement can cause significant psychological damage, including cognitive delays, increased suspicion and paranoia, increased anxiety, fear, aggression and hostility, heightened feelings of helplessness and depression, and increased thoughts and attempts at self-mutilation and suicide. Many prisoners held in this severe form of isolation are juveniles or individuals with serious mental illness, to whom it is particularly damaging. Although solitary confinement is common in the rest of the world, nowhere is it more prevalent as a long-term prisoner management tool than in the United States. US courts have found that solitary confinement is a violation of the Eighth Amendment to the US Constitution in certain situations, yet the practice persists. As a global movement against solitary confinement grows, the United Nations and regional human rights tribunals have spoken out against the practice. A robust body of international case law has defined the contours of when solitary confinement is cruel, inhuman or degrading treatment, and the instances in which it is torture. International bodies prohibit solitary confinement for juveniles, prisoners with mental illness, and prisoners on death row or with life sentences. International tribunals generally find solitary confinement for all prisoners contrary to applicable law where it constitutes incommunicado detention, where it is unnecessarily prolonged without justification, and where the totality of conditions of confinement cross a threshold into unacceptable cruelty. As international law prohibiting solitary confinement crystallizes, the practice in the United States may be curtailed through reliance on international law by US judges. Further, the US executive may take an increased interest in curbing solitary confinement to avoid reputational damage among the global community.