Updated: August 18, 2018

This is the term, in both sf and popular culture, used to indicate sentient extraterrestrial beings – creatures from other worlds endowed with reason, consciousness, thought, Intelligence (the terms for and conceptions of this vital but slippery quality vary). Aliens may have minds somewhat less capable than ours, of comparable capacity, of greater (even vastly greater) power, or minds so different that comparison becomes impossible. They may appear as invaders (see Invasion) or teachers, as allies or enemies, as victims of human exploitation or judges of human civilization, as Secret Masters guiding human history (see Uplift), or as utterly indifferent forces paying no attention to humanity at all. Aliens may look like us, resemble (more or less) any number of Earthly species, or take on shapes we have never seen or imagined, forms so strange we sometimes fail to recognize them (and they us) as fellow beings at all.

While the overwhelming majority of aliens hail from outer space, aliens may occasionally emerge from little-explored zones on our own planet, from Under the Sea or the rarefied layers of the upper atmosphere. They may be found Underground or in the hollow interior of our planet (see Hollow Earth). Some aliens come from other Dimensions, while others have their origins in Parallel Worlds or Alternate Histories. In most (though not all) cases, non-extraterrestrial aliens function in sf texts in much the same way that extraterrestrials do, and their nearer origins make little difference.

Approaches to the subject in sf fall into two broad (though often overlapping) categories. On one hand, writers have long exploited the rich literary opportunities offered by the figure of the alien, whether or not the aliens they imagine differ radically from human beings or other Earthly models. Occasionally they have striven to convey the sense of an alien mind which thinks in ways unlike those of human beings. (This has frequently been acknowledged as one of the most difficult challenges an sf writer can face.) On the other hand,writers have variously attempted to depict truly alien beings, the products of environments and histories unlike Earth's, sometimes even based on physical substrates other than familiar matter (e.g., radio waves or stellar plasma). The very possibility of an alien intelligence is an existential provocation, the stuff of identity crisis, and writers of fiction have been exploring the implications of alien beings since their existence began to be seriously considered in the Proto SF of the early seventeenth century. Over that time – and particularly since the turn of the twentieth century – writers have expanded and complicated our conception of alien biologies and societies, deployed aliens in critiques of the chauvinistic and parochial assumptions embedded in human culture, and carried on a complex intertextual meditation on the likely effects of an encounter with alien beings on human consciousness and civilization.