An alien invasion is an unexpected and uninvited visit to another planet, commonly Earth in fiction, by aliens. Invasions usually involve the use of a "mothership" and other UFOs. In films and television, invasions are usually a custom of "evil" aliens who are on a conquest to "take-over-the-world". In other cases, an invasion is a good thing or simply an alien's visit to the planet Earth. The "alien invasion" phenomenon is extremely popular in science fiction, with near 100 examples of the subject in literature, film, etc.