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 very liberally interpreted, so that a law will generally not be overturned if there is even a remote connection to a constitutionally-given power, usually the power to regulate interstate commerce.

The Federal government has found clever ways to circumvent this amendment. One common way is liberally interpret the interstate commerce clause so that acts which have even the slightest potential effect on interstate commerce can be regulated by the federal government. Federal drug laws and gun laws use this justification. Another way is to deny states federal funding if certain state laws do not conform to Federal guidelines. The national 55 mph 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" around and on public school campuses was struck down because there was no clause in the Constitution authorizing it. The opinion did not mention the Tenth Amendment.