In United States v. Lopez (1995), the Supreme Court ruled that Congress had exceeded its constitutional authority under the Commerce Clause when it passed a law prohibiting gun possession in local school zones. The case arose out of the Gun-Free School Zones Act of 1990, which made it a federal offense "for any individual knowingly to possess a firearm at a place that the individual knows, or has reasonable cause to believe, is a school zone." Alfonso Lopez Jr., a high school senior, was convicted in a federal district court for knowingly possessing a concealed handgun and bullets at his San Antonio high school. The Fifth Circuit Court of Appeals reversed the decision, ruling that the law was beyond the reach of Congress's power under the Commerce Clause. The government appealed to the Supreme Court, which reviewed the case in 1994.

The Supreme Court, in a 5-4 decision written by Chief Justice William Rehnquist and released in 1995, ruled that the Gun-Free School Zones Act was unconstitutional and overturned Lopez's conviction. The Court ruled that the act exceeded the limited powers of Congress under the Constitution, rejecting the government's argument that the act was constitutional because the buying and selling of guns and associated illicit activities affect "interstate commerce," which Congress may regulate under the Commerce Clause. The government claimed that gun violence in schools leads to both more dangerous and thus less commercially healthy neighborhoods and less economically productive kids. The Court responded that regulating guns in local schools is not sufficiently related to Congress's Commerce Clause power to pass constitutional muster.

The Court maintained that Congress could constitutionally regulate three things under the Commerce Clause: instrumentalities of commerce, the use or channels of commerce, and activities that substantially