US investigations into UFOs include:

The Interplanetary Phenomenon Unit (IPU), established by the US Army sometime in the 1940s, and about which little is known. In 1987, British UFO researcher Timothy Good received a letter confirming the existence of the IPU from the Army Director of Counter-intelligence, in which it was stated, "… the aforementioned Army unit was disestablished during the late 1950s and never reactivated. All records pertaining to this unit were surrendered to the U.S. Air Force Office of Special Investigations in conjunction with operation BLUEBOOK." The IPU records have never been released.

Project Blue Book, previously Project Sign and Project Grudge, conducted by the United States Air Force from 1947 until 1969

The secret U.S. Army/Air Force Project Twinkle investigation into green fireballs (1948–1951)

Ghost rockets investigations by the Swedish, U.K., U.S., and Greek militaries (1946–1947)

The secret CIA Office of Scientific Investigation (OS/I) study (1952–53)

The secret CIA Robertson Panel (1953)

The secret USAF Project Blue Book Special Report No. 14 by the Battelle Memorial  Institute (1951–1954)

The Brookings Report (1960), commissioned by NASA

The public Condon Committee (1966–1968)

The private, internal RAND Corporation study (1968)

The private Sturrock Panel (1998)

Thousands of documents released under FOIA also indicate that many U.S. intelligence agencies collected (and still collect) information on UFOs, including the Defense Intelligence Agency (DIA), FBI, CIA, National Security Agency (NSA), as well as military intelligence agencies of the Army and Navy, in addition to the Air Force.

The investigation of UFOs has also attracted many civilians, who in the U.S formed research groups such as National Investigations Committee On Aerial Phenomena (NICAP, active 1956–1980), Aerial Phenomena Research Organization (APRO, 1952–1988), Mutual UFO Network (MUFON, 1969–), and Center for UFO Studies (CUFOS, 1973–).