The term ‘ghost guns’—derived from the fact that these firearms are un-serialized, difficult to trace, and often remain invisible to the tracking and regulation covering traditionally manufactured firearms—refers to a wide range of homemade or improvised firearms. Assembled from parts, including those developed via 3D-printing technology, or from kits that include unfinished pieces (typically the receiver of the gun), ghost guns require the purchaser to be proficient in only basic machining to make the gun functional. Current federal firearms regulations do not require manufacturers of these parts or unfinished pieces or those assembling them to include serial numbers because the unassembled parts are not considered firearms. Thus, ghost gun parts and kits can be purchased online without being subject to most firearm regulations. Ghost guns present unique challenges to law enforcement agencies and make traditional investigative techniques less effective. As discussions about ghost guns, their regulation, and concerns for public safety migrate to the public sphere, this fact sheet provides five key points around the assembly, proliferation, and identification of ghost guns.