This revised Fact Sheet explores the mandate and functioning of the Working Group on Arbitrary Detention, established in 1991 by the United Nations Commission on Human Rights to investigate cases of arbitrary deprivation of liberty. The publication presents the various components of the Working Group’s mandate, its procedures and operation, in addition to some of its relevant jurisprudence. It also explains how to submit a case to the Working Group and the criteria used by the Working Group to determine whether a detention is arbitrary. It further provides practical information on how to submit allegations of arbitrary detention and the relevant forms.