Book excerpt: Thousands of well-documented reports of strange and baffling events, without known causes, grouped under the amorphous heading UFOs, are sufficient, in the authors' view, to suggest that they merit serious scientific study. It is their opinion that a scientific study, seriously undertaken, would reveal cause-effect relationship for some, possibly all, of the phenomena that constitute the UFO enigma. The accumulated evidence for the UFO enigma consists of reports from many parts of the world. These reports contain at least nineteen categories of human experiences ranging from simple observations of strange lights to accounts of abductions by strange, unknown beings. Most reports are from ordinary persons unable to identify a natural cause for what they have observed or experienced. Many of these are sufficiently well-documented to form the basis for scientific studies. The core of the UFO enigma is found in the thousands of challenging reports from highly intelligent, knowledgeable persons with sufficient scientific expertise and experience to enable them to make clear distinctions between events with and without known causes. These latter reports form the basis of this book's assertion that the UFO enigma can, and should be studied scientifically. This book is the authors' attempt to provide an overview of the UFO enigma as a scientific problem and to suggest sources of information and methods for attempting to solve it.