Over the past decades, the increasing participation of private actors in the provision of security has gained more and more academic attention in Belgium. This scholarly evolution has led to advanced criminological debate, theory and concepts on the trajectory of frontline policing activities of the private security industry, such as manned guarding. However, it has largely neglected the development of a significant yet traditionally ignored sub-sector of private security: the private investigation sector. With this innovative historical criminological research, we aim to explore post-war developments in the private investigation sector in Belgium for the first time. Our focus lies on identifying long-term trends in its policy, regulation and nature from 1945 until present, in order to advance substantial new understandings on less-visible historical and contemporary forms of the private contribution to policing. To achieve these objectives, this interdisciplinary research combines well-elaborated historical criminological methods and policy theory, which has proven its major theoretical and analytical worth. While historical sources are incorporated for addressing present-day criminological debates, the Multiple Streams Approach is used to understand how the issue of private investigations has been repeatedly rising and falling on the policy agenda over time. The findings will make an important contribution to the field of criminology, history, law, and political sciences.