International law is consistent in affirming that the deprivation of liberty should be a “last resort” for children. This norm is affirmed by the extensive evidence that detention is detrimental to the wellbeing and healthy development of young people. Yet while it is broadly understood that detention of children and youth should be uncommon, there is much less clarity around what the “last resort” mandate means in practice—that is, under what circumstances is detention permissible and what, if anything, must states do prior to considering the detention of a young person. Drawing on scholarship on criminal justice and human rights, the work of international treaty bodies, and other human rights sources, this Article explores the meaning of “last resort” under international law, focusing in particular on the use of arrest, detention, and imprisonment in the youth justice context. The Article then proposes a framework for operationalizing the “last resort” mandate so that governments can respond in a more rights-affirming manner to children who are in conflict with the law.