Building on the analysis of state obligations provided in Chap. 2 , this chapter explores the determination of state responsibility in relation to disappearances committed by non-state actors, based on the rules enshrined in the Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA) and an analysis of jurisprudence of the European and Inter-American human rights courts (ECtHR and IACtHR) and United Nations treaty bodies. The chapter begins by examining under what conditions disappearances committed by non-state actors can be directly attributed to the state, based on the general criteria for attribution established in the ARSIWA, and then, more specifically, based on the notions of ‘authorisation, support or acquiescence’ contained in the IHRL definition of ‘enforced disappearance’ as lex specialis to the general rules. This includes a review of how ‘acquiescence’ has been established by UN treaty bodies, the ECtHR and the IACtHR and an analysis of the definition provided by the Committee on Enforced Disappearances. Second, for cases where a violation cannot be directly attributed to a state, the latter can still be responsible for failing to meet its positive obligations to prevent and respond. Therefore, the chapter proceeds to analyse approaches by regional and international human rights bodies to determining state responsibility in cases involving non-state actors as perpetrators of disappearances or other human rights violations. The analysis focuses mainly on the determination of responsibility for a failure to prevent and on case law by the ECtHR and IACtHR, as there is little case law of this nature by UN treaty bodies. The chapter ends with a description of the two prevalent approaches used, based either on the state’s awareness of a risk to a specific victim, or the source of risk to a wider group of potential victims, and highlights uncertainties regarding disappearance cases arising from a conflation of preventive and investigative obligations.