Both the Ukrainian government authorities and Russia-backed separatists in eastern Ukraine have held civilians in prolonged, arbitrary detention, without any contact with the outside world, including with their lawyers or families. In some cases, the detentions constituted enforced disappearances, meaning that the authorities in question refused to acknowledge the detention of the person or refused to provide any information on their whereabouts or fate. Most of those detained suffered torture or other forms of ill-treatment. Several were denied needed medical attention for the injuries they sustained in detention. Amnesty International and Human Rights Watch investigated in detail nine cases of arbitrary, prolonged detention of civilians by the Ukrainian authorities in informal detention sites and nine cases of arbitrary, prolonged detention of civilians by Russia-backed separatists. This report details cases that took place mostly in 2015 and the first half of 2016.