International humanitarian law and international human rights law seek to prevent people from going missing, and to clarify the fate and whereabouts of those who do go missing while upholding the right to know of their relatives. When implementing international law at the domestic level, national authorities should plan carefully before engaging in any policy or legal reform that will address the issue of missing persons and the response to the needs of their families. This article seeks to present a general overview of the provisions of international law that are relevant to understanding the role of national implementation vis-à-vis the clarification of the fate and whereabouts of missing persons and the response to the needs of their relatives. It also presents the role that the ICRC has played in this regard and highlights three challenges that may arise at the national level when working on legal and policy reforms.