States constitutionally impose individual income taxes on two bases: (1) Residency: a state of residence can tax its residents and domiciliaries and (2) Source: the state in which income is earned can tax the individual earner. At present, there is no articulated constitutional barrier to double taxation of individual income. That is, there is no requirement that the source state and residence state collaborate to tax no more than 100% of an individual\u27s income, and there is no requirement that only one state consider itself the source of a particular item of income. In the realm of corporate income taxation the Commerce Clause has been interpreted as requiring each state to tax only the income fairly apportioned to that state. This article argues that the commerce clause applies to check the discriminatory taxation of individual income as well. In addition, the article synthesizes the goals of the Privileges and Immunities Clause with those of the dormant Commerce Clause, and demonstrates that the Privileges and Immunities Clause does not occupy the field to the exclusion of the dormant Commerce Clause. In fact, even absent application of the dormant Commerce Clause, the article posits a separate, arguably more fundamental rationale to avoid double taxation: Both the Article IV and the 14th Amendment Privileges and Immunities Clauses share the animating constitutional principle of national unity and true national citizenship. Retaliatory and protectionist taxing regimes undermine the interest in national unity, and should fall under these clauses
