This Article examines whether and how the doctrine of intergovernmental tax immunity should be interpreted to apply between the several states. First, it traces the history and development of the doctrine of intergovernmental tax immunity from its origins and early broadening to its narrowing, the development of divergent standards or state immunity from federal taxation, and federal immunity from state taxation. It then discusses more recent Supreme Court cases which address the doctrine of intergovernmental tax immunity as well as relevant and analogous immunities under the Tenth and Eleventh Amendments. After tracing the Supreme Court jurisprudence, it analyzes the few cases which have addressed the existence of intergovernmental tax immunity between the several states. Against this background, it concludes that intergovernmental tax immunity constitutionally prohibits one state from imposing a property tax on another state, but that excise and privilege taxes may be imposed by one state upon another. The Article illustrates how this issue may arise, for example, if one state attempted to impose a state-level Unrelated Business Income Tax (UBIT) upon the income earned by another state, and conclude that the standard developed herein appropriately resolves such a dispute.