Downham v. Alexandria was a United States Supreme Court case that dealt with the issue of whether a state could tax the income of a non-resident. The case was brought by a resident of the District of Columbia, who owned property in the state of Virginia. The plaintiff argued that the state of Virginia had no right to tax his income from the property, as he was not a resident of the state. The Supreme Court ultimately ruled in favor of the plaintiff, holding that the state of Virginia could not tax the income of a non-resident. The Court reasoned that the power to tax was an attribute of sovereignty, and that the state of Virginia had no authority to tax the income of a non-resident. The Court further held that the power to tax was limited to the state in which the taxpayer resided, and that the state of Virginia had no authority to tax the income of a non-resident. This decision established the principle that states cannot tax the income of non-residents, and has been cited in numerous subsequent cases.

In Downham v. Alexandria, the Supreme Court was asked to decide whether a tax imposed by the City of Alexandria on all real estate within its jurisdiction violated the Constitution's Contract Clause. The majority opinion held that it did not violate this clause because there was no contract between the city and any particular taxpayer that would be impaired by such a tax. However, Justice Field dissented from this decision, arguing that while there may have been no express contract between taxpayers and their local government regarding taxes, an implied contract existed in which citizens had an expectation of reasonable taxation for services provided by their municipality. He argued further that when taxes are so high as to amount to confiscation or oppression they become unconstitutional under both state and federal law; thus he concluded that if Alexandria's tax rate exceeded what could reasonably be expected for municipal services then it should be struck down as unconstitutional under both state and federal law.