An extraterrestrial or alien is a lifeform that did not originate on Earth. (The word extraterrestrial means 'outside Earth'.) Extraterrestrials are a common theme in modern science-fiction, and also appeared in much earlier works such as the second-century parody True History by Lucian of Samosata.

The 2nd century writer of satires, Lucian, in his True History claims to have visited the Moon when his ship was sent up by a fountain, which was peopled and at war with the people of the Sun over colonisation of the Morning Star.

The way people have thought about extraterrestrials is tied to the development of actual sciences. One of the first steps in the history of astronomy was to realize that the objects seen in the night sky were not gods or lights, but physical objects like Earth. This notion was followed by the one that celestial objects should be inhabited as well. However, when people thought about such extraterrestrials, they thought of them simply as people, indistinguishable from humans. As people had never considered a scientific explanation for the origin of mankind or its relation with other lifeforms, any hypothetical rational lifeforms had by necessity to be humans. Even in mythology, all deities are mostly humanlike. For example, Voltaire's Micromégas (1752) features people from Saturn, who are simply of higher proportions. Johannes Kepler's Somnium (1634), Francis Godwin's The Man in the Moone (1638), Cyrano de Bergerac's Les estats et empires de le lune (1657) and others all thought of selenites that differ from humanity only in culture or habits. Few writers ventured beyond anthropomorphic designs, some exceptions were Bergerac's Les estats et empires du soleil [fr] and Miles Wilson's The History of Israel Jobson, the Wandering Jew (1757).

This was changed by the 1859 book On the Origin of Species by Charles Darwin, which proposed the theory of evolution. This book caused a revolution in fiction as much as it did in science, as authors began to imagine extraterrestrial races completely different from human beings. With the rationale that evolution in other worlds may take completely different directions than on Earth, aliens began to be described as a-human creatures. Usually, authors used features from other animals, such as insects, crabs, and octopuses. One of the first works featuring genuinely alien lifeforms was Camille Flammarion's non-fiction book Les mondes imaginaires et les mondes reels (1864) and his novel Lumen (1887). He described sentient trees, tentacled seal-like creatures pushing against a harsh atmosphere, and life made of silicon and magnesium. Some other aliens are the octopean Martians from H. G. Wells's The War of the Worlds (1898), the Selenites from Wells's The First Men in the Moon (1901), the birdlike Tweel from Stanley G. Weinbaum's A Martian Odyssey (1934) and even a sentient star in Olaf Stapledon's Star Maker (1937). However, most aliens in works of the era were still basically humans, as the Martians from Hugh MacColl's Mr. Stranger's Sealed Packet (1889), Robert Cromie's A Plunge into Space (1890), and the Venusians from Milton Worth Ramsey's Six Thousand Years Hence (1891).

The War of the Worlds not only used Darwinian evolution to explain its non-humanoid aliens, but also explored the implications of the theory of evolution towards alien lifeforms. Martians appear as an apex predator above even humans, a threat to the survival of the species. However, they struggle against Earth's higher gravity and thicker atmosphere, for which they were not adapted to, and eventually succumb to simple bacteria, as they lack immunity to them. The story also worked as a critique of British imperialism, by inverting it, and introduced the tropes of the alien invasion and the depiction of extraterrestrials as monsters. Wells also wrote The First Men in the Moon, the first attempt to describe in detail the workings of an alien civilization. He based the roles of the Selenites in those of an ant colony, although those roles are more the result of social structures rather than genetic design. However, his work still relied in satire and had more in common with Jonathan Swift's Gulliver's Travels (1726) than with the alien civilizations seen in later science fiction works.

The new literary genre of science fiction explored both extraterrestrials and space exploration, as in From the Earth to the Moon (1865) and Around the Moon (1870) by Jules Verne.

Pulp magazines emerged as a new venue for science fiction. Many stories were set in worlds with quasi-human aliens, menaced by dangerous monsters and beautiful women serving as a love interest for the hero. This is the pattern of Ralph Milne Farley's The Radio Man (1924) and others. Pulps also featured monstrous alien invaders, in the style of The War of the Worlds. In the first space operas, such as those from Amazing Stories, good and evil aliens were clearly distinct: spider-like, octopoid and most reptilian aliens were villains, and humanoid, mammalian and birdlike aliens were the good ones. It was also frequent for the classic trope of the alien invasion to be inverted, with humans conquering alien worlds instead; such stories were usually unapologetically genocidal.

Most aliens in pulp magazines originated from planets or moons of the Solar System, mainly Martians, Venusians, Jovians, and Mercurians. Aliens from Neptune and the moons of Jupiter and Saturn also appeared but were rare. The humanoid type was still the most frequent type of alien, despite evolution being fully accepted in the scientific community by this point. Stanley G. Weinbaum made a significant change in A Martian Odyssey (Wonder Stories), by designing a Martian ecosystem with native creatures, unlike the plants or animals from Earth. Such creation was largely free of satire, melodrama and other frequent tropes of the genre.