The role of the expert in criminal justice proceedings has long been an area of contention and debate among practitioners and scholars. Yet, there has been little to no discussion of the role of experts within the International Criminal Court (ICC). Here a bifurcation is drawn between those experts that are witnesses and the non-witness experts that are 'hidden' from the official processes, yet play a role in the investigations and analysis of cases. The focus of this piece is on the Office of the Prosecutor's (OTP) (hired) external non-witness experts. To date, there has been little to no criminological attention to this phenomenon. Yet there are organic concerns such as the process of knowledge-making, objectivity, 'truth' and substantive and procedural concerns that merit attention given that they are contracted to provide expertise in a particular area to help inform the way in which evidence is analyzed with the broader goal of 'proving' something important in a trial. Additionally, there is institutional equality of arms concerns related to the use of these non-disclosed experts that directly relates to due process. The use of non-witness experts is further complicated by the multiplicity of and/or relevance of fields from which they are drawn. It is the goal of this paper to explore these issues and provide a frame for theoretically situating the impact of non-witness external experts on the judicial process within the context of the ICC.