In this chapter I examine the evolution of UN peace operations, focusing particularly on the place of policing. The chapter identifies substantive change in UN policing, as well as the increasing importance attached to police and rule of law initiatives as part of UN peace operations. As such, it lays the foundations for the inquiry in the chapters that follow. The chapter proceeds in three parts. Firstly, it traces the trajectory of quantitative, qualitative and normative change in the peace operations architecture at the UN from their inception to the present day. This section highlights the move towards multidimensional missions and points to the reality that they are engaged in early peacebuilding efforts. The second part charts the evolving role of police within peace operations and their place in emerging thinking that places resurrecting the rule of law as a cornerstone of war-to-peace transitions under the auspices of UN multidimensional peace operations. The section demonstrates that as UNPOL have become increasingly central, they have also become ever more involved in intrusive reform activities at the peacekeeping-peacebuilding nexus that implicate a broader security and rule of law system. The third part highlights key challenges relating to generating and deploying missions as well as implementing mandates once in theatre. In particular, this section argues that the UNPOL apparatus faces significant hurdles due to structural constraints to recruitment of sufficient quantity and quality of UNPOL and unproven assumptions underpinning the modalities and even the ethic of the UNPOL project. It is argued that these enduring and emerging issues have significant implications for the impact of UNPOL. The chapter concludes by identifying organisational learning and monitoring and evaluation as key avenues for addressing these challenges.