An unidentified flying object (UFO), or unidentified anomalous phenomenon (UAP), is any perceived airborne, submerged or transmedium phenomenon that cannot be immediately identified or explained. Upon investigation, most UAPs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.

Project Blue Book was the code name for the systematic study of unidentified flying objects by the United States Air Force from March 1952 to its termination on December 17, 1969. The project, headquartered at Wright-Patterson Air Force Base, Ohio, was initially directed by Captain Edward J. Ruppelt and followed projects of a similar nature such as Project Sign established in 1947, and Project Grudge in 1949. Project Blue Book had two goals, namely, to determine if UFOs were a threat to national security, and to scientifically analyze UFO-related data.