On 19 September 2016, IOM officially became a “related organization” in the UN system. This development challenges the received wisdom in much of the literature on IOM, which has generally assumed that the agency has thrived precisely because it was outside the UN system, unfettered by principled commitments and obligations. If so, why would IOM undercut its own comparative advantage? This chapter explores the evolution of the IOM-UN relationship, the factors that prompted IOM’s entry into the UN system, and the agreement through which this was achieved. Some have questioned whether as a related organization IOM is really part of the UN system. In contrast, this chapter argues that IOM is indeed now part of the UN system, and brings into focus the diversity of shifting opinions on this issue among IOM officials and member states. The chapter contends that the decision to join the UN system reflects a fluctuating set of cost-benefit calculations, with recent IOM leaders concerned about protecting their turf as the leading international agency focused on migration, and convinced that IOM had become intertwined with and dependent on the UN, without reaping the benefits of membership in the UN system. The possibility of IOM formally entering the UN system seemed largely theoretical until 2015, when a confluence of factors dramatically accelerated the process. However, this chapter contends that this development needs to be understood in the context of the long history of cooperation-and competition-between IOM and the UN, with close collaboration in the humanitarian sphere providing a foundation for recasting the IOM-UN relationship. Just as IOM’s humanitarian engagements are critical to the organization’s expansion and internal tensions, they are also central to apprehending this institutional realignment.