The Second Amendment of the United States Constitution reads: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." Such language has created considerable debate regarding the Amendment's intended scope. On the one hand, some believe that the Amendment's phrase "the right of the people to keep and bear Arms" creates an individual constitutional right for citizens of the United States. Under this "individual right theory," the United States Constitution restricts legislative bodies from prohibiting firearm possession, or at the very least, the Amendment renders prohibitory and restrictive regulation presumptively unconstitutional. On the other hand, some scholars point to the prefatory language "a well regulated Militia" to argue that the Framers intended only to restrict Congress from legislating away a state's right to self-defense. Scholars have come to call this theory "the collective rights theory." A collective rights theory of the Second Amendment asserts that citizens do not have an individual right to possess guns and that local, state, and federal legislative bodies therefore possess the authority to regulate firearms without implicating a constitutional right.

In 1939 the U.S. Supreme Court considered the matter in United States v. Miller. 307 U.S. 174. The Court adopted a collective rights approach in this case, determining that Congress could regulate a sawed-off shotgun that had moved in interstate commerce under the National Firearms Act of 1934 because the evidence did not suggest that the shotgun "has some reasonable relationship to the preservation or efficiency of a well regulated milita . . . ." The Court then explained that the Framers included the Second Amendment to ensure the effectiveness of the military.

This precedent stood for nearly 70 years when in 2008 the U.S. Supreme Court revisited the issue in the case of District of Columbia v. Heller (07-290). The plaintiff in Heller challenged the constitutionality of the Washington D.C. handgun ban, a statute that had stood for 32 years. Many considered the statute the most stringent in the nation. In a 5-4 decision, the Court, meticulously detailing the history and tradition of the Second Amendment at the time of the Constitutional Convention, proclaimed that the Second Amendment established an individual right for U.S. citizens to possess firearms and struck down the D.C. handgun ban as violative of that right. The majority carved out Miller as an exception to the general rule that Americans may possess firearms, claiming that law-abiding citizens cannot use sawed-off shotguns for any law-abiding purpose. Similarly, the Court in its dicta found regulations of similar weaponry that cannot be used for law-abiding purposes as laws that would not implicate the Second Amendment. Further, the Court suggested that the United States Constitution would not disallow regulations prohibiting criminals and the mentally ill from firearm possession.

Thus, the Supreme Court has revitalized the Second Amendment. The Court continued to strengthen the Second Amendment through the 2010 decision in McDonald v. City of Chicago (08-1521). The plaintiff in McDonald challenged the constitutionally of the Chicago handgun ban, which prohibited handgun possession by almost all private citizens. In a 5-4 decisions, the Court, citing the intentions of the framers and ratifiers of the Fourteenth Amendment, held that the Second Amendment applies to the states through the incorporation doctrine. However, the Court did not have a majority on which clause of the Fourteenth Amendment incorporates the fundamental right to keep and bear arms for the purpose of self-defense. While Justice Alito and his supporters looked to the Due Process Clause, Justice Thomas in his concurrence stated that the Privileges and Immunities Clause should justify incorporation.

However, several questions still remain unanswered, such as whether regulations less stringent than the D.C. statute implicate the Second Amendment, whether lower courts will apply their dicta regarding permissible restrictions, and what level of scrutiny the courts should apply when analyzing a statute that infringes on the Second Amendment. As a general note, when analyzing statutes and ordinances, courts use three levels of scrutiny, depending on the issue at hand:

Recent lower-court case law since Heller suggests that courts are willing to uphold

regulations which ban weapons on government property. US v Dorosan , 350 Fed. Appx. 874 (5th Cir. 2009) (upholding defendant’s conviction for bringing a handgun onto post office property);

, 350 Fed. Appx. 874 (5th Cir. 2009) (upholding defendant’s conviction for bringing a handgun onto post office property); regulations which ban the illegal possession of a handgun as a juvenile, convicted felon. US v Rene , 583 F.3d 8 (1st Cir. 2009) (holding that the Juvenile Delinquency Act ban of juvenile possession of handguns did not violate the Second Amendment);

, 583 F.3d 8 (1st Cir. 2009) (holding that the Juvenile Delinquency Act ban of juvenile possession of handguns did not violate the Second Amendment); regulations which require a permit to carry concealed weapon. Kachalsky v County of Westchester , 701 F.3d 81 (2nd Cir. 2012) (holding that a New York law preventing individuals from obtaining a license to possess a concealed firearm in public for general purposes unless the individual showed proper cause did not violate the Second Amendment.)

More recently, the Supreme Court reinforced its Heller ruling in its Caetano v. Massachusetts (2016) decision. The Court found that the lower "Massachusetts Supreme Judicial Court was wrong in the three reasons it offered for why the state could ban personal possession or use of a “stun gun” without violating the Second Amendment." The Supreme Court, however, remanded the case without further instructions, so this per curiam ruling did not do much to further clarify the Supreme Court's stance on the Second Amendment.

See constitutional amendment.