Although the 1949 Geneva Conventions have now been universally ratified, the same is not the case for other treaties of international humanitarian law. As a result, customary international humanitarian law continues to be relevant today. This was clearly recognized in 1995 when States party to the Geneva Conventions mandated the ICRC to carry out a study on customary rules of international humanitarian law applicable in international and non-international armed conflicts. This study was published in 2005 after widespread research and consultations. This article takes stock of the impact of the study and discusses some of the salient features of the methodology of assessing custom. It also briefly describes the current effort to update the practice underlying the Study.