In Brushaber v. Union Pacific Railroad Co., the Supreme Court of the United States declares the federal income tax constitutional.

Brushaber v. Union Pacific Railroad Co., 240 U.S. 1 (1916), was a landmark United States Supreme Court case in which the Court upheld the validity of a tax statute called the Revenue Act of 1913, also known as the Tariff Act, Ch. 16, 38 Stat. 166, enacted pursuant to Article I, section 8, clause 1 of, and the Sixteenth Amendment to, the United States Constitution, allowing a federal income tax. The Sixteenth Amendment had been ratified earlier in 1913. The Revenue Act of 1913 imposed income taxes that were not apportioned among the states according to each state's population.

The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all U.S. federal court cases, and over state court cases that turn on questions of U.S. constitutional or federal law. It also has original jurisdiction over a narrow range of cases, specifically "all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party." In 1803, the court asserted itself the power of judicial review, the ability to invalidate a statute for violating a provision of the Constitution via the landmark case Marbury v. Madison. It is also able to strike down presidential directives for violating either the Constitution or statutory law.

An income tax is a tax imposed on individuals or entities (taxpayers) in respect of the income or profits earned by them. Income tax generally is computed as the product of a tax rate times the taxable income. Taxation rates may vary by type or characteristics of the taxpayer and the type of income.