This book contains many of the most significant cases in UFO history. A UFO is defined as a mysterious object seen in the sky for which it is claimed no orthodox scientific explanation can be found, often supposed to be a vehicle carrying extraterrestrials. Scientists have tried to provide logical explanations for most UFO reports. For example, in many cases, a reported UFO later was identified as a meteor, a planet, a rocket, a star, an artificial satellite, or a weather balloon. Aircraft or their exhaust trails, seen under unusual lighting conditions, have also been reported as UFOs. In addition, atmospheric conditions may produce optical illusions that are described as UFOs. While reading this book, it must be understood that these are only unsolved UFO cases and therefore these in no way support the existence of UFOs. It is nonetheless very important to find suitable scientific evidence before drawing any conclusion on UFOs.