This revision contains corrections together with an index and a list of references. Millions of people believe that there are mysterious objects flying in the sky. They were originally described as Unidentified Flying Objects - hence UFOs - but have more recently been termed UAPs - Unidentified Anomalous Phenomena.There are many more sightings of UAPs over land, than have been associated with maritime UFOs, which have been termed Unidentified Submersible Objects (USOs). The abbreviation UAP is now used to cover both UFOs and USOs. UAPs seem to fall mainly into 2 distinct categories.They may be objects which are apparently metallic in construction, of many shapes and which can vary in size from larger than a car to being absolutely massive. Alternatively they can be balls, either of some form of plasma, or metallic, giving off or reflecting light, and varying from the size of a tennis ball upwards. This book looks at UFO sightings worldwide, and a t the various options for their origin. It looks at the possibility of alien bases, at their frequency and distribution, and their likely purpose..