The existence of alien (extraterrestrial) beings has been one of the enduring themes of science fiction.
One of the first fiction writers to use Alien beings from another planet was H.G. Wells. The War of the Worlds is the best known of his works; it also introduced the modern reader to the recurring concept of interplanetary invasion by malign aliens.
The alien invasion was one of two themes which was to crop up again and again in the 'pulp science fiction' years, the 1930s to 1960s. The Cold War made people particularly receptive to the idea of evil and incomprehensible beings coming to destroy or enslave earthly (usually American) life. Examples of these include the short story The Liberation of Earth by William Tenn.
The contrasting picture of aliens during this time was that of the wise and civlised race coming to Earth to impart their wisdom and solve our problems. These stories were almost as popular as the invasion theme, at a time when mankind looked as though it was in the brink of destroying itself. A good example of this story is the movie The Day the Earth Stood Still.
One of the most frequently portrayed alien races from our own solar system are the Martians, Mars being the most romanticized of the other planets whose surface conditions are closest to being amenable to life. See Mars in fiction for more details on the red planet's numerous roles.
Many of the aliens have been hostile and alien invasion has been a very popular idea in Anglo-Saxon science fiction.
Writers have created a long list of extraterrestrial creatures and intelligent beings; see the list of aliens in fiction.