Extraterrestrial life is life that does not originate from Earth. It is also called alien life, or, a "space alien". These as-yet-hypothetical life forms range from simple bacteria-like organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist, there is no unambiguous evidence for its existence so far. The science of extraterrestrial life is known as exobiology. The science of astrobiology also considers life on Earth as well, and in the broader astronomical context. Meteorites that have fallen to Earth have sometimes been examined for signs of microscopic extraterrestrial life. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earthwas about 400 million years old. According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe." Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial intelligence, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction. Over the years, science fiction works, especially Hollywood's involvement, has increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try to get in contact with life in outer space, whereas others argue that it might be dangerous to actively call attention to Earth. This book discusses the latest theories, estimates and facts that surround the possibilities of extraterrestrial life in its various states and forms and is designed to be a reference and provide an overview of the topic and give the reader a structured knowledge to familiarize yourself with the topic at the most affordable price possible. The accuracy and knowledge is of an international viewpoint as the edited articles represent the inputs of many knowledgeable individuals and some of the most current knowledge on the topic, based on the date of publication.