This paper offer a general theoretical framework that can, first, help integrate the different explanations of child marriage and, second, guide the development of measurement tools indispensable for child marriage M&E. Our theoretical framework is based on insights into how individuals make decisions. The collective practice of child marriage is ultimately a cluster of individual behaviors, so that, if we want to understand it, we have to understand why individuals behave in certain ways. This paper was written in collaboration with UNICEF and benefited from the technical input of UNICEF staff.