In Maryland State Comptroller of the Treasury v. Wynne, the Court could reshape core features of dormant Commerce Clause law. Maryland’s theory in the case is that it can lay an income tax on every penny of an individual resident’s income even if some of that income is earned entirely outside the state and therefore, in keeping with standard state practice, already taxed elsewhere. On its face, this approach exposes interstate income earners to overlapping income taxation. Maryland’s scheme thus violates a cardinal principle of dormant Commerce Clause law, “forbidding” state laws that expose interstate commerce “to the risk of a double tax burden to which intrastate commerce is not exposed.” Indeed, this principle applies with the greatest possible force in this case because Maryland does not merely expose taxpayers engaged in interstate commerce to the risk of a double tax burden or even “serious concerns of double taxation.” Maryland essentially guarantees double taxation by taxing its residents’ out-of-state income in all-out fashion while giving only partial credit for taxes paid elsewhere on that same income. Nor can Maryland defend its taxing system on the ground that, although unorthodox, it is “evenhanded.” It is not evenhanded because Maryland, while positing that residence alone should count with regard to taxing the income of Marylanders, simultaneously taxes non-Marylanders when they generate income in Maryland. When it comes to taxing interstate activity, Maryland thus seeks to have its cake and eat it too. In such circumstances, if any, the overriding safeguard against “multiple taxation” of interstate income must and does apply.