Both as scientists and as devotees of science fiction, we have long been interested in space travel. When reports of unidentified flying objects began to increase in the years between 1947 and 1952, one of us (D.H.M.) collected and studied the limited information available about the sightings. He soon concluded (with a slight feeling of disappointment!) that the flying saucers were not vehicles from other worlds but were only mundane objects and events of various kinds, some of them commonplace, some familiar chiefly to meteorologists, physicists, and astronomers. At a conference with Air Force officials in Washington in April 1952, he presented his idea that planetary mirages, sundogs, reflections, and other astronomical, atmospheric, and optical phenomena probably accounted for a large percentage of the mysterious UFOs. This suggestion met with strong skepticism from some of the conferees who at that time were sympathetic to the interplanetary hypothesis and were, of course, better acquainted with military than with physical science. Other conferees, however, wished to consider and test the theories offered. Proof obviously required a knowledge of all the facts of a given sighting, facts that often were not available to the public. The Air Force therefore granted access to the file of UFO cases. At the same time, since many of the cases were then classified as secret, the Air Force imposed the condition that security regulations must be strictly observed. D.H.M. was then preparing a book to present his explanations of flying saucers. Acceptance of the Air Force offer, with the accompanying restriction, would have prevented his publishing analyses based on material in the files. It would also have hindered any future public discussion of the UFO problem. For these reasons he felt compelled to decline the opportunity. In the spring of 1959 as we began planning the present book, we again requested permission to study the Air Force records of UFO sightings. This time the officials generously opened their files to us without restriction. Thus we have been able to include detailed studies of particular incidents, to give the explanations found for most of them by Air Force investigators, to explain the causes of some hitherto unsolved cases, and to suggest highly probable solutions for several classic “Unknowns.” To discuss each one of the thousands of unidentified flying objects reported during the last fifteen years is obviously impossible. We have therefore chosen to describe the common types of sighting and to analyze some of the representative and most interesting cases in each category. In general we have avoided using the names of the persons involved; but when the names are well known to the flying-saucer public and have previously appeared in print, we have felt no obligation to disguise them.