This thematic report by the Office of the United Nations High Commissioner for Human Rights (OHCHR) examines: 1) the scale of detention in the context of the armed conflict in eastern Ukraine by Government actors and armed groups and other actors in territory controlled by the self-proclaimed ‘Donetsk people’s republic’ and the self-proclaimed ‘Luhansk people’s republic’ from 14 April 2014 to 30 April 2021; 2) the prevalence and patterns of conflict-related arbitrary detention, including secret and incommunicado detention; 3) the prevalence and patterns of conflict- related torture and ill-treatment, including conflict-related sexual violence; and 4) accountability for these violations, including remedy and reparation to the victims. The report also presents two emblematic case studies of conflict-related arbitrary detention, torture and ill-treatment, one in the Kharkiv regional department of the Security Service of Ukraine (Annex I) and the second in the ‘Izoliatsiia’ detention facility in armed group-controlled Donetsk (Annex II). Cases of arbitrary detention, torture and ill-treatment in the Autonomous Republic of Crimea and the city of Sevastopol, Ukraine, occupied by the Russian Federation that took place following the occupation and are still ongoing are out of the scope of the present report. These cases have been covered in OHCHR periodic reports on the human rights situation in Ukraine and thematic reports on the situation of human rights in Crimea, as well as in the reports of the United Nations Secretary-General on the situation of human rights in Crimea.