The determination of taxing rights vis-à-vis multinational corporations is a two-step process. The application of the nexus rule is the first step, and the application of one or more profit allocation rules is the second step. The nexus rule is a ‘yes-no’ decision rule; it determines whether a tax administration has the right to levy taxes on a given multinational group member operating within its borders based on the materiality of the group member’s economic presence therein. The profit allocation rules, taken together, determine the amount of individual firms’ consolidated pre-tax profits to attribute to each jurisdiction satisfying the nexus rule. In the OECD’s view, the sweeping economic changes that digitalization and globalization more broadly set in motion—more precisely, the subset of these changes that the OECD deems salient for tax purposes—has significantly eroded the foundations of the international tax system, and both the nexus rule and the long-standing profit allocation rules have lagged well behind the new economic realities. The OECD’s Pillar One-Pillar Two Solution and the associated special purpose nexus rule, described in this chap., purport to bring the international tax system into the twenty-first century and meet this moment thereby.