The Roswell incident is the 1947 recovery of balloon debris from a ranch near Corona, New Mexico by United States Army Air Forces officers from Roswell Army Air Field, and the conspiracy theories, decades later, claiming that the debris involved a flying saucer and that the truth had been covered up by the United States government. On July 8, 1947, Roswell Army Air Field issued a press release stating that they had recovered a "flying disc". The Army quickly retracted the statement and said instead that the crashed object was a conventional weather balloon.The Roswell incident did not surface again until the late 1970s, when retired lieutenant colonel Jesse Marcel, in an interview with ufologist Stanton Friedman, said he believed the debris he retrieved was extraterrestrial. Ufologists began promoting a variety of increasingly elaborate conspiracy theories, claiming that one or more alien spacecraft had crash-landed and that the extraterrestrial occupants had been recovered by the military, which then engaged in a cover-up.

In 1994, the United States Air Force published a report identifying the crashed object as a nuclear test surveillance balloon from Project Mogul. A second Air Force report, published in 1997, concluded that stories of "aliens bodies" probably stemmed from test dummies being dropped from high altitude.

Conspiracy theories about the event nevertheless persist, and the Roswell incident continues to be of interest in popular media. The incident has been described as "the world's most famous, most exhaustively investigated, and most thoroughly debunked UFO claim".The city of Roswell, New Mexico has capitalized on the event; the city's official seal now features a little green man while the city contains countless ufology attractions, events, statues and iconography.

An unidentified flying object (UFO) is any perceived aerial phenomenon that cannot be immediately identified or explained. On investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.

Scientists and skeptic organizations such as the Committee for Skeptical Inquiry have provided prosaic explanations for a large number of claimed UFOs being caused by natural phenomena, human technology, delusions, or hoaxes. Small but vocal groups of "ufologists" favour unconventional, pseudoscientific hypotheses, some of which go beyond the typical extraterrestrial visitation claims and sometimes form part of new religions.

While unusual sightings have been reported in the sky throughout history, UFOs did not achieve their current cultural prominence until the period after World War II, escalating during the Space Age. The 20th century saw studies and investigations into UFO reports conducted by governments (such as Projects Grudge and Sign in the United States, and Project Condign in the United Kingdom), as well as by organisations and individuals.