Federalism is a distinctive form of constitutional rule but one that has largely been neglected by both political and constitutional theory. Existing accounts of federalism tend to focus almost exclusively upon its institutional manifestation. What is lacking is an account of the common conceptual underpinnings that unite these various institutional forms within the genus of one constitutional idea. In this chapter Stephen Tierney argues that the core idea of federalism can only be arrived at by way of constitutional theory. Constitutional theory explains both how and why law is used to manage political power. Federal constitutions manage and transform political power for a discrete purpose that is fundamentally distinguishable from other constitutional forms. This chapter contends that federalism must be addressed as a specific genus of constitutional government for the modern state which, in the act of constitutional union, gives foundational recognition and accommodation to the state’s constituent territorial pluralism. The purpose of the federal constitution is to maintain the foundational relationship between pluralism and union through the creation and reconciliation of different orders of government. This marks a significant fork in the road between federal and unitary constitutionalism, not just in institutional terms but at the most fundamental level of constitutional identity and legitimacy.