Amendment X (the Tenth Amendment) of the United States Constitution, which is part of the Bill of Rights, states:

The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.

The Tenth Amendment is generally recognized to be a truism. In United States v. Sprague (1931) the Supreme Court noted that the amendment "added nothing to the [Constitution] as originally ratified". That said, it makes explicit the idea that the federal government is limited only to the powers it is granted in the Constitution. However, this amendment is narrowly interpreted, so that a law will generally not be overturned if there is even a remote connection to a constitutionally-given power, often the power to regulate interstate commerce.

Many that favor a more strict interpretation of the Constitution believe that the Federal government has used clever 'loopholes' to circumvent this amendment. The use of the interstate commerce clause to justify federal laws regarding narcotics and guns and the use of the general welfare clause to justify social programs are among the areas of contention. Another controversial technique Congress has used is to deny states federal funding if certain state laws do not conform to federal guidelines. For example, the national 55 mph (89 km/h) speed limit and the national 21-year drinking age were imposed through this method; the states would lose highway funding if they refused to pass such laws.

In United States v. Lopez, , a federal law mandating a "gun-free zone" on and around public school campuses was struck down because there was no clause in the Constitution authorizing it. The opinion did not mention the Tenth Amendment.