The UN Working Group on Arbitrary Detention (WGAD) is a UN human rights body specializing on arbitrary deprivation of liberty, one of the most grave human-rights violations under international law. The WGAD, established in 1991 by UN Commission on Human Rights resolution 1991/42, is a UN human-rights special procedure, which prefers the non-legal, problem-solving approach to cases of human-rights violations. Thanks to the favorable political winds at the time of its creation, the WGAD adopted the quasi-judicial procedure for individual petitions, common among the human-rights treaty-monitoring bodies whose main duty is to authoritatively interpret and apply the respective treaties. The hybrid nature of the WGAD contributed to its expansive remedies for arbitrary detention and progressive development of the norms related to personal liberty. Because the WGAD renders authoritative legal opinions on the basis of established norms and procedure in response to individual communications, the WGAD has been recognized as a quasi-judicial organ not only by the international treaty-monitoring bodies but also by regional human rights courts. At the same time, unlike the treaty bodies, the WGAD’s quasi-judicial proceedings are unencumbered by the requirements for the ratification of treaties by concerned states or the exhaustion of domestic rules thanks to the flexible, non-judicial procedure employed by other UN special procedures. The only applicable condition is indeed the occurrence of arbitrary deprivation of liberty. The WGAD has expanded the doctrinal scope of personal liberty with the concept of arbitrary detention lato sensu to address the detention of political prisoners under convenient legal pretexts as well as to expand its competence, which has to be occasioned by arbitrary deprivation of liberty. Over many centuries of struggle for freedom, national constitutions universally recognize the liberty of person and require its deprivation to be in accordance with the law to protect individuals from the whims of state power. However, the mass deprivation of liberty by Nazi Germany and its totalitarian allies under the color of law prompted the post-war international human-rights law to demand not only the legal basis but also the absence of arbitrariness. The 1948 Universal Declaration of Human Rights, the 1966 International Covenant on Civil and Political Rights and the 1988 Body of Principles for the Protection of All Persons under Any Form of Detention or Imprisonment as well as other international and regional instruments incorporated the prohibition of arbitrary arrest and detention. International law codified the right to be informed of the reasons for arrest and charges, judicial oversight of criminal detention, the general assumption against pre-trial detention, habeas corpus and an enforceable right to compensation for unlawful detention. While these rules may be effective in revising incompatible domestic regulations or practice, their efficacy was limited for arrest and detention of political prisoners and minorities that formally hued to the law. The WGAD’s institutional peculiarities helped it overcome the limitations posed by the existing international law, and provide more effective remedies to the victims of arbitrary detention. In general, the work of monitoring bodies may be categorized as (1) the review function to determine the compliance of states, (2) the correction function to indicate remedies or ways to prevention reoccurrence in case of violations, and (3) the creative function to provide detailed norms to be applied in particular instances. The human rights treaty-monitoring bodies and UN human rights special procedures, the two pillars of international human-rights system, began to develop from the 1960s. The treaty-monitoring bodies are adapt at performing review and creative functions as it is their role to interpret and apply the relevant treaties through quasi-judicial consideration of individual petitions and states parties’ periodic reports as well as the issuance of general comments/recommendations. However, the treaty bodies can exercise its competence only for those states that had consented to it and, for the individual communications, only after the exhaustion of domestic remedies. By contrast, the UN special procedures, created by UN bodies under the authority derived from the UN Charter, adopt their own methods of work and pragmatically employ pragmatic, non-judicial discussion of individual petitions with governments, urgent appeals and country visits. The UN special procedures are free from adherence to a particular treaty or its provisions, but generally lack the quasi-judicial mandate. The prevailing post-Cold War optimism at its inception gave the WGAD the explicit power to investigate individual cases of arbitrary detention. The WGAD interpreted this power to create quasi-judicial proceedings for individual communications modeled after the practice of the Human Rights Committee, the monitoring body for the International Covenant on Civil and Political Rights. By adopting the flexible procedural rules typical for UN special procedures dispensing with the need for the prior acceptance of its competence by the offending state or the exhaustion of domestic remedies by the victims, the WGAD had the best of both worlds. The Working Group in effect established “un habeas corpus universel,” according to its former members, or universal habeas corpus. The WGAD did face an existential threat from some member states of the UN Commission on Human Rights when they took issue with the WGAD’s application of the International Covenant on non-party states and finding of arbitrary detention for post-conviction imprisonment of political prisoners. The WGAD made nominal concessions to apply only the Universal Declaration to the non-party states and to exclude imprisonment under court judgments from its purview provided that they were compatible with international standards; in fact, the WGAD’s position was consolidated. The jurisprudence and reports of the WGAD may not be said to be legally binding per se, but they embody the progressive formation of international customary and soft law related to liberty of person by their authoritative interpreter. The WGAD’s contribution of the substantive law includes its use of five legal categories of arbitrary deprivation of liberty that encompass not only deprivation of liberty without legal basis (Category I) but also deprivation of liberty resulting from the exercise of other rights and freedoms (Category II), deprivation of liberty rendered arbitrary by grave violations of due process rights (Category III), prolonged administrative detention of foreign nationals without administrative or judicial recourse (Category IV) and deprivation of liberty constituting illegal discrimination under international law (Category V). The resulting arbitrary detention lato sensu substantially broadens the scope of arbitrary deprivation of liberty. The WAGD also broadened deprivation of liberty to include house arrest and taking refuge in a foreign embassy surrounded by police. The Working Group’s jurisprudence held the prohibition of arbitrary detention to be peremptory and erga omnes norms and reiterated that widespread or systematic imprisonment or other severe deprivation of physical liberty in violation of fundamental rules of international law may constitute crimes against humanity. Not surprisingly, the WGAD interpreted the legal effect of reservations to or derogation from the International Covenant that restrict the liberty of person in a restrictive manner. The achievements made thus far notwithstanding, it is incumbent upon the WGAD, as a uniquely empowered UN special procedure, to continue its dynamic interpretation of substantive and procedural rules to remedy and prevent arbitrary deprivation of liberty.