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and it is valuable when a single sighting is described by more than one witness. The likelihood of hoax is decreased if the witnesses were unknown to each other before the sighting. In some cases an account may be supported by various forms of supplementary evidence. There are many cases in which photographs have been taken while a UFO was witnessed by several apparently competent observers. Holes have been left in the ground where a UFO had supposedly landed or vegetation has been damaged or on fire. Occasionally (rarely), radioactivity has been detected. In one case a fence was magnetized where a UFO had passed over it. Many strange samples have been left, such as liquid residues, "angel's hair," and other materials. In no case, of course are these things by themselves conclusive, since virtually any sort of evidence could be fraudulently produced. We remain dependent upon the reliability of the witnesses, but sometimes these secondary evidences can contribute to an evaluation of the sighting. Many radar sightings of UFOs are on file. In a few cases, a UFO has been simultaneously observed by radar and by witnesses, both on the ground and in an aircraft. Menzel and Boyd (1963) have clearly pointed out, however, that radar evidence is far from positive proof. There are many natural atmospheric and other phenomena as well as imperfections in radar instruments which can produce so- called radar angels. We must consider the argument from both sides, however. Just because radar angels are not necessarily UFOs, we are still not entitled to conclude that any unusual blip on the screen is a radar angel: We should certainly not conclude that UFOs cannot be extraterrestrial spaceships, because if they were, our radar net would pick them up. The fact of the matter is, our radar net does pick up many returns which are not identifiable in terms of known aircraft (e.g. apparent objects moving several thousand miles per hour through the atmosphere). Many of these are undoubtedly radar angels, in the true sense of the word, but we can't say that some are not spaceships from Mars! A secondary form of supporting evidence is that of pattern. While Sagan (1963) fails to see any pattern because of the noise, other investigators feel that many patterns can be established from the reports. Michel studied the sightings in France in 1954 and found that occasionally they appeared to fall upon great circle arcs of the Earth's surface (Michel, 1958). It is extremely difficult (Menzel, 1964; Vallee, 1964) to evaluate the significance of such a pattern. In many cases, the lines could be If all of these criteria are met for a given UFO report, then it is highly likely that we are not dealing with a conventional object misinterpreted. The detail usually precludes this. In such a case, the UFO could be an extraterrestrial spaceship or it could fit into one of the categories discussed below. Can the UFOs be pure figments of the mind - hallucinations, dreams, and the like? Probably there are cases in which this is the proper explanation, but it is a difficult one to apply to situations in which many witnesses describe with reasonable uniformity a single UFO. In such cases, the psychological explanation would have to fall back on areas such as extrasensory perception, which are really not due purely to chance. III. Psychological Phenomena