In 2008, the U.S. Supreme Court decided District of Columbia v. Heller , in which the Court held that the Second Amendment to the U.S. Constitution protects an individual right to possess a firearm, unconnected with service in a militia, and the use of that firearm for traditionally lawful purposes, such as self-defense within the home. Shortly afterward in McDonald v. City of Chicago , the Supreme Court held that the Second Amendment also applies to the states, but it did not further explore the scope of the Second Amendment. Although Heller did not constitute "an exhaustive historical analysis ... of the full scope of the Second Amendment," the Court noted that its decision "does not imperil every law regulating firearms," and "[does] not cast doubt [] on longstanding regulatory measures [such] as 'prohibitions on the possession of firearms by felons and the mentally ill,' 'laws forbidding the carrying of firearms in sensitive places such as schools and government buildings, or laws imposing conditions and qualifications on the commercial sale of arms.'" Since Heller and McDonald , both federal and state firearms laws have been regularly challenged under the Second Amendment. This report first discusses the standard of judicial review that the lower courts generally have fashioned to determine if a firearm law is in violation of the Second Amendment. Next, the report examines select categories of firearms laws that have been challenged under the Second Amendment. These include (1) prohibitions on certain persons based on age and on criminal history; (2) state concealed carry laws; and (3) state and local assault weapons bans. An examination of these categories could provide some insight as to how courts might assess future firearms legislation on Second Amendment grounds. In the 113 th Congress, several gun control proposals have been introduced. These include, among others, measures that would ban certain assault weapons and prohibit possession of large capacity magazines (e.g., S. 150 / H.R. 437 / S. 33 ); and measures that would require background checks on private transfers of firearms or ammunition (e.g., S. 22 / S. 174 / S. 374 / H.R. 141 ). A significant question left open by the Court in Heller centers on the standard of scrutiny that should be applied to laws regulating the possession and use of firearms. Generally, there are three levels of judicial scrutiny. First, strict scrutiny, the most rigorous, requires a statute to be narrowly tailored to serve a compelling state interest. Second, intermediate scrutiny, requires a statute to further a government interest in a way that is substantially related to that interest. Third, the rational basis standard merely requires the statute to be rationally related to a legitimate government function. The Court in Heller rejected the rational basis test and also explicitly rejected Justice Breyer's "interest-balancing" inquiry, distinguishing this "judge-empowering" approach from the "traditionally expressed levels" of scrutiny. However, the Court did not establish or clearly apply any judicial standard, declaring instead that the challenged firearms provisions were unconstitutional "[u]nder any of the standards of scrutiny that [the Court has] applied to enumerated constitutional rights." After Heller , some lower courts seemingly did not perform an extensive analysis or apply a particular standard of scrutiny in determining that a challenged federal firearms law was valid under the Second Amendment. Rather, they analogized the challenged firearms provisions to those listed as "presumptively lawful" in Heller and found them to be constitutional. Under this approach, courts have upheld bans on possession by felons, by substance abusers, by illegal aliens, and by people convicted of domestic violence. Interestingly, some of these categories, as well as those not listed in the Court's dicta as "presumptively lawful regulatory measures," are not necessarily "longstanding," as they were enacted in the 20 th century. As touched upon below, courts have wrestled with how to interpret and incorporate the "presumptively lawful" language from Heller with an analytical framework that involves application of means-end scrutiny. However, the U.S. Court of Appeals for the Third Circuit (Third Circuit), in United States v. Marzzarella , attempted to establish a framework for how to evaluate firearms laws that did not fall within those identified as "presumptively lawful." The statute at issue in Marzzarella was 18 U.S.C. Section 922(k), which is the federal ban on the possession of unmarked firearms. In the Third Circuit's view, there are two possibilities of how the "presumptively lawful" categories are to be treated. The first possibility is that these types of firearms regulations could be those which regulate "conduct outside the scope of the Second Amendment," meaning that they would not be subject to any heightened judiciary scrutiny. Or, the language "may suggest the restrictions are presumptively lawful because they pass muster under any standard of scrutiny." The Third Circuit favored its first interpretation, finding it more consistent based on the text and structure of the Heller decision. With this view, the Third Circuit noted that Heller suggested a two-step approach: "First, we ask whether the challenged law imposes a burden on conduct falling within the scope of the Second Amendment's guarantee (citations omitted). If it does not, our inquiry is complete. If it does, we evaluate the law under some form of means-end scrutiny. If the law passes muster under the standard, it is constitutional. If it fails, it is invalid." With respect to scope, the Court in Heller seemed to indicate that the "core" Second Amendment right protects "the right of law-abiding citizens to possess non-dangerous weapons for self-defense in the home." It is less clear, however, the types of regulation that might burden conduct protected by the Second Amendment. The defendant, Marzzarella, argued that firearms without serial numbers must come within the scope of the Second Amendment because firearms in common use in 1791 did not have serial numbers. The Third Circuit rejected this because it found that "it would make little sense to categorically protect a class of weapons [under the Second Amendment] bearing a certain characteristic wholly unrelated to their utility." The court was further skeptical of the defendant's argument that "possession in the home is conclusive proof that §922(k) regulates protected conduct [under the Second Amendment]." Due to a lack of historical evidence, however, the court could not conclude with certainty that the Second Amendment did not protect possession of unmarked firearms in the home. The Third Circuit, therefore, proceeded to assume that the federal ban burdened the defendant's Second Amendment right and examined the law under "some form of means-end scrutiny." Looking to First Amendment jurisprudence for guidance, the court noted that even an enumerated, fundamental right may be subjected to varying levels of scrutiny depending on the circumstances. The court chose to apply an intermediate scrutiny standard to evaluate Section 922(k) finding that, similar to content-neutral time, place, and manner restrictions on speech, the firearms provision also regulated the manner in which persons could lawfully exercise their Second Amendment rights. Although the intermediate scrutiny standard in the First Amendment context is articulated in different ways, "[t]hey all require the asserted governmental end to be more than just legitimate, either 'significant,' 'substantial,' or 'important," and require "the fit between the challenged regulation and the asserted objective be reasonable, not perfect." With respect to Section 922(k), the Third Circuit held that the statute passes muster under intermediate scrutiny because it does not "severely limit the possession of firearms," and reasonably fits to achieve the government's substantial and important interest in preserving serial numbers for tracing purposes. As discussed below, many courts generally have employed the Third Circuit's two-part inquiry when determining if a challenged federal or state firearms provision violates the Second Amendment, including those the Court in Heller determined to be presumptively lawful. Both federal and state laws have long prohibited certain categories of individuals from possessing firearms. Congress enacted the Gun Control Act of 1968 (GCA or Act) to "keep firearms out of the hands of those not legally entitled to possess them because of age, criminal background, or incompetency, and to assist law enforcement authorities in the states and their subdivisions in combating the increasing prevalence of crime in the United States." The GCA establishes a comprehensive statutory scheme that regulates the manufacture, sale, transfer, and possession of firearms and ammunition. For instance, the GCA prohibits federal firearms licensees (or licensed dealers) from selling handguns to any person under the age of 21 and long guns (i.e., rifles and shotguns) to any person under the age of 18. Licensed dealers are also subject to several requirements designed to ensure that a firearm is not transferred to an individual disqualified from possession under the act. Under federal law there are nine categories of persons who prohibited from possessing, receiving, or transferring a firearm. The individuals targeted by this provision include (1) persons convicted of a crime punishable by a term of imprisonment exceeding one year; (2) fugitives from justice; (3) individuals who are unlawful users or addicts of any controlled substance; (4) persons legally determined to be mentally defective, or who have been committed to a mental institution; (5) aliens illegally or unlawfully in the United States, as well as those who have been admitted pursuant to a nonimmigrant visa; (6) individuals who have been discharged dishonorably from the Armed Forces; (7) persons who have renounced United States citizenship; (8) individuals subject to a pertinent court order; and, finally, (9) persons who have been convicted of a misdemeanor domestic violence offense. The following section reviews judicial decisions that have evaluated the constitutionality of the federal age requirement to purchase firearms, as well as the federal prohibition on misdemeanants of domestic violence from possessing firearms.