Chapter 6 uses the Paris Peace Conference of 1919–1920 as a vantage point for demonstrating the consequences for international politics of the new, largely colonial boundary expertise of the late nineteenth century. While existing accounts of the Conference emphasize interactions between Wilsonian ideas and power-political realities, focusing on boundary experts illuminates a set of concerns separate from both. Modern polities rely on experts to construct and maintain boundaries, and thus such experts can at least potentially exert various types of influence in territorial politics, through mechanisms of persuasion, delegation, and instrumentalization. The chapter shows how each of these mechanisms was at work as boundary experts pushed the Paris Peace Conference towards mountain boundaries, particularly in the case of Czechoslovakia’s boundaries, the Austro-Yugoslav boundary, and Italy’s boundaries with Austria and Yugoslavia.