Reports concerning crimes and abuses committed by peace operations personnel deployed in situations of humanitarian crisis often hit the news. In these instances, troop-contributing States (TCSs) retain both a prerogative and a duty to undertake criminal investigations as prescribed by international law. To investigate effectively, however, a TCS will need to interact with the other actors involved in a peace operation: the host State, the leading international organization, and other TCSs. When a local is killed, for example, the investigating TCS must seek the authorization of the host State in order to perform an autopsy on the victim’s body, to avoid infringing the latter’s sovereignty. In this sense, partial or total non-cooperation on the part of the host State may hamper a TCS’s ability to conduct an effective investigation. The present article examines the obligation to investigate in the context of peace operations, particularly as deriving from human rights law and international humanitarian law (IHL). It further explores how the specific agreements applicable in UN and NATO operations address investigative cooperation, aiming to map relevant normative gaps. The article finally proposes an interpretation de lege ferenda of existing obligations under human rights law and IHL, according to which the duty to investigate would require TCSs (a) to regulate investigative cooperation with the other actors involved in a peace operation and (b) to ensure that such actors conduct investigations in line with the required standards of effectiveness.