A federal firearms license (FFL) is a license in the United States that enables an individual or a company to engage in a business pertaining to the manufacture or importation of firearms and ammunition, or the interstate and intrastate sale of firearms. Holding an FFL to engage in certain such activities has been a legal requirement within the United States since the enactment of the Gun Control Act of 1968. The FFL is issued by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (BATFE, commonly known as the "ATF")

The federal firearms license was established to and implement the Gun Control Act of 1968. The 1968 act was an update or revision of the Federal Firearms Act of 1938 (FFA), which required all manufacturers and dealers of firearms who ship or receive firearms or ammunition in interstate or foreign commerce to have a license, and forbade them from transferring any firearm or most ammunition to any person interstate unless certain conditions were met. As a practical matter, this did not affect the interstate commerce in firearms or ammunition, because the 1938 FFA lacked a provision specifically restricting "prohibited purchasers" as defined in the FFA from purchasing firearms under false pretenses/documentation (i.e. those who would otherwise have been considered "prohibited purchasers" were still able to get away with firearms purchases by providing falsified identity information; since background checks as understood today did not yet exist, convicted felons and other "prohibited purchasers" would still buy firearms unhindered by using fake identification). It was with the adoption of the Gun Control Act (GCA) in 1968, which repealed most of the FFA, that the lawful interstate trade of firearms was limited almost entirely to persons holding a federal firearms license in the United States. As background checks were available by this time, enforcement of the GCA via the bottleneck provided by background checks and the FFL requirement was more effective than that of the FFA.

Certain types of firearms, accessories and other weapons are currently restricted under the National Firearms Act (NFA). In addition to a current FFL (of whatever "type" is applicable), the ATF requires that business owners who are planning to import, manufacture or deal in restricted materials also pay a Special Occupational Tax, or "SOT" (thereby making the business owner a "Special Occupational Taxpayer").

C&R firearms are defined in Title 27, Code of Federal Regulations, Part 478.11 as those "of special interest to collectors by reason of some quality other than is associated with firearms intended for sporting use or as offensive or defensive weapons." To be recognized by ATF as a C&R firearm, a firearm must fall into at least one of the following three categories:

C&R firearms include most manually operated and semi-automatic firearms used by a military force prior to 1975. This includes most firearms used by the warring nations in World War I and World War II. However, the firearm must normally also be in its original configuration to retain the C&R designation. So, for example, an unaltered Mauser Karabiner 98k rifle used by the German Army in World War II is a C&R firearm – but the same rifle "sporterized" with new stock and finish is generally not considered a C&R firearm. There is an ambiguous point in how the license is currently administered, in that some firearms altered by the militaries that issued them have been confirmed by the ATF to retain C&R status, though whether this applies to all such conversions (the examples given by the ATF were the Spanish M1916 Guardia Civil, FR-7, and FR-8 Mausers) also remains ambiguous. However, as long as the receiver (the part of the firearm that is regulated by the ATF) is over 50 years old, the firearm qualifies as a Curio & Relic – ATF states explicitly that, in addition to newer firearms it individually approves, firearms automatically achieve C&R status upon turning 50, provided they are in the original configuration. If modified significantly, the 50-year clock resets to the date of modification. (Specific examples are available in the ATF FAQs.) Certain automatic firearms have been designated as C&R firearms, and a C&R may be used to acquire these as well.

Collectors may acquire C&R firearms in interstate commerce, e.g., via mail or phone order or the Internet, or in person. (This is especially important for collectors of pistols and revolvers since they may not otherwise be acquired outside a collector's state of residence.) Collectors are not considered to be FFL dealers and have no special privileges concerning non-C&R firearms, nor may they "engage in the business" of regularly selling C&R firearms to persons who do not have an FFL. Selling of C&R firearms does not require an FFL transfer across state lines, only if the firearm has a collectible status. The purpose of the C&R license is to enable a collector to acquire C&R firearms for their personal collection and not to become a firearms dealer.

(D) At the election of a licensed collector, the annual inspection of records and inventory permitted under this paragraph shall be performed at the office of the Attorney General designed for such inspections which is located in closest proximity to the premises where the inventory and records of such licensed collector are maintained.

Any firearm sold as a C&R firearm once changed out of its original configuration cannot be resold as a C&R firearm.In regard to conversions; certain pistols have been approved for sale with added safety conversions (i.e. Polish and Romanian Tokarev pistols, to which a manual safety was added to meet import requirements). Certain other modifications, such as period sporterisations, are arguably C&R qualified as they represent the gun culture of the period. An example would be a Lee–Enfield or 98K Mauser military rifle that had been converted into a continental style sporter before World War II. These common conversions occurred more than 50 years ago, and represent a sub-type of special interest to collectors.