The UFO phenomenon (standing for Unidentified Flying Object) consists of reports of unusual flying objects that remain unidentified after scientific inquiry. It first came to public attention in the United States in 1947, when a pilot reported seeing nine unusual objects flying in formation in the state of Washington. Since 1947, the U.S. federal government, private research institutions, and individual scientists have collected data about the phenomenon. Although UFOs are not a phenomenon unique to the United States, American organizations and private individuals have taken the lead in collecting, analyzing, and publishing sighting reports.

Nearly all research efforts have determined that a small but significant number of sightings remain "unidentified" after scientific investigation. This is especially true with reports made by the most articulate witnesses and containing the most data. Although the primary objective of private UFO researchers was to collect and analyze reports, they also sought to convince the public and the scientific community of the legitimacy of the subject. Their task was made all the more difficult by the perceived unlikelihood of the extraterrestrial phenomenon, and in part by publicity hungry charlatans and self-promoters ("contactees") who, beginning in the 1950s, made fictitious claims about meeting "space brothers" and traveling to distant planets, or hinted darkly about secret government conspiracies with aliens.

In the twenty-first century, the UFO phenomenon persisted, apparently unaffected by societal events. It continued to maintain a ubiquitous presence in popular culture, researchers continued to study it, and, although scientists and academics still scorned it, ordinary people continued to report both sightings and abduction accounts.