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 1948. Project Blue Book had two goals, namely, to determine if UFOs were a threat to national security, and to scientifically analyze UFO-related data.

Thousands of UFO reports were collected, analyzed, and filed. As a result of the Condon Report, which concluded that the study of UFOs was unlikely to yield major scientific discoveries, and a review of the report by the National Academy of Sciences, Project Blue Book was terminated in 1969. The Air Force supplies the following summary of its investigations:

No UFO reported, investigated, and evaluated by the Air Force was ever an indication of threat to our national security;

There was no evidence submitted to or discovered by the Air Force that sightings categorized as "unidentified" represented technological developments or principles beyond the range of modern scientific knowledge; and

There was no evidence indicating that sightings categorized as "unidentified" were extraterrestrial vehicles.

By the time Project Blue Book ended, it had collected 12,618 UFO reports, and concluded that most of them were misidentifications of natural phenomena (clouds, stars, etc.) or conventional aircraft. According to the National Reconnaissance Office a number of the reports could be explained by flights of the formerly secret reconnaissance planes U-2 and A-12. 701 reports were classified as unexplained, even after stringent analysis. The UFO reports were archived and are available under the Freedom of Information Act, but names and other personal information of all witnesses have been redacted.