The elimination of child marriage is a goal that ranks high on the agendas of civil society organizations, national governments, and multilateral institutions. To date, however, there has been very little scholarship on the historical debates over the definition of child marriage. This article examines the history of age-restricted marriage as it was debated during the development of human rights instruments in the post-World War II era. Using archives of the United Nations and affiliated organizations, we detail how and why (...) efforts to establish a universal minimum age for marriage were repeatedly unsuccessful. We illustrate how the current, commonly used definition of child marriage—as marriage before the age of 18—has never been agreed upon in a legally binding international convention but was seemingly established through a discursive innovation that borrowed principles from the 1989 Convention on the Rights of the Child. (shrink)