UN human rights mechanisms continue to proliferate, producing numerous decisions and voluminous reports. This article reviews the ways in which such mechanisms apply international humanitarian law, including the law of Geneva and the law of The Hague. In doing so, it focuses mainly on the practice of the rapporteurs appointed by the UN Commission on Human Rights to investigate the human rights situations in specific countries and on that of the thematic rapporteurs and working groups which the Commission has entrusted with monitoring specific types of serious human rights violations wherever they occur, in particular the Special Rapporteur on Extrajudicial, Summary or Arbitrary Executions and the Representative of the Secretary-General on Internally Displaced Persons, whose mandates most often lead them to examine abuses occurring in the context of armed conflicts. Reference is also made to two innovative mechanisms which functioned in El Salvador: the first UN-sponsored “truth commission” and the first human rights monitoring body established as part of a comprehensive mechanism for monitoring compliance with a UN-sponsored peace agreement. Certain observations made by treaty monitoring bodies are also mentioned.