This study is the first scientific attempt, on the basis of global data, to comprehend the magnitude of the situation of children deprived of liberty, its possible justifications and root causes, as well as conditions of detention and their harmful impact on the health and development of children. The study also identifies best practices in non-custodial solutions applied by States in relation to the following six situations: (a) detention of children in the administration of justice; (b) children living in prisons with their primary caregivers; (c) migration-related detention; (d) deprivation of liberty in institutions; (e) detention in the context of armed conflict; and (f) on national security grounds. The study proposes recommendations to support States and the United Nations in dealing with this phenomenon.