Under what conditions do UN peace operations assist the International Criminal Court by sharing information, providing logistical assistance, and executing arrest warrants? Even though international criminal tribunals often have to rely on UN peace operations to implement their mandate, we know preciously little about when and why UN peace operations decide to assist. This dissertation sets out to answer this question by combining insights from the international criminal justice, peacekeeping, and inter-organizational relations literature to build a theoretical framework that goes beyond the legal focus of much of the current literature on UN-ICC interactions. Taking the leadership of UN peace operations as its unit of analysis, it investigates the effects of structural conditions (mandate, international support, capacity, and precedents), the intermediate risk context (as it relates to stability, local legitimacy, and force protection), and agent-level factors. To test this theoretical framework, the thesis present a single-country case study of assistance provided by the UN mission in the DRC and a plausibility probe of other peace operations in ICC situation countries. It relies on new empirical material, including over 130 interviews and comprehensive archival research. With this material, the thesis sheds light on how the UN navigates the terrain of conflict mediation and punitive accountability and provides insights into the collaborative but contingent relationship between the UN and the ICC. It shows that the UN encountered serious dilemmas when being asked to assist the UN, frequently having to weigh their willingness to assist against potential negative repercussions on stability, consent, and impartiality. In explaining how the UN responds to these dilemmas, the dissertations highlights the role of international support for assistance, the potential risks of assistance to stability and local legitimacy, and the impact that individuals can have in navigating structural constraints and responding to risks.