In Pollock v. Farmers' Loan & Trust Co. the Supreme Court of the United States declares unapportioned income tax to be unconstitutional.

Pollock v. Farmers' Loan & Trust Company, 157 U.S. 429 (1895), affirmed on rehearing, 158 U.S. 601 (1895), was a landmark case of the Supreme Court of the United States. In a 5–4 decision, the Supreme Court struck down the income tax imposed by the Wilson–Gorman Tariff Act for being an unapportioned direct tax. This decision was superseded in 1913 by the Sixteenth Amendment to the United States Constitution, which allows Congress to levy income taxes without apportioning them among the states.

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.

The United States federal government and most state governments impose an income tax. They are determined by applying a tax rate, which may increase as income increases, to taxable income, which is the total income less allowable deductions. Income is broadly defined. Individuals and corporations are directly taxable, and estates and trusts may be taxable on undistributed income. Partnerships are not taxed, but their partners are taxed on their shares of partnership income. Residents and citizens are taxed on worldwide income, while nonresidents are taxed only on income within the jurisdiction. Several types of credits reduce tax, and some types of credits may exceed tax before credits. Most business expenses are deductible. Individuals may deduct certain personal expenses, including home mortgage interest, state taxes, contributions to charity, and some other items. Some deductions are subject to limits, and an Alternative Minimum Tax (AMT) applies at the federal and some state levels.