First contact is a common theme in science fiction about the first meeting between humans and extraterrestrial life, or of any sentient species' first encounter with another one, given they are from different planets or natural satellites. It is closely related to the anthropological idea of first contact

Alien invasion or space invasion is a common feature in science fiction stories and film, in which extraterrestrial lifeforms invade the Earth to exterminate and supplant human life, enslave it, harvest people for food, steal the planet's resources, or destroy the planet altogether. It can be considered as a science-fiction subgenre of the invasion literature, expanded by H. G. Wells's seminal alien invasion novel The War of the Worlds, and is a type of "first contact" science fiction.