The terraforming (literally, "Earth-shaping") of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth in order to make it habitable by humans. The term is sometimes used broadly as a synonym for planetary engineering in general. The concepts of terraforming are rooted both in science fiction and actual science. The term was probably coined by Jack Williamson in a science-fiction story published in 1942 in Astounding Science Fiction, but the actual concept pre-dates this work. Olaf Stapledon's Last and First Men (1930) provides an example in fiction in which the planet Venus is modified after a long and destructive war with the original inhabitants of the planet.

Humans currently do not possess the technological or economic means to terraform another planet or moon. Since space exploration is in its infancy, terraforming techniques remain speculative. Based on experiences with Earth, the environment of a planet can be altered in a deliberate way; however the feasibility of creating an unconstrained planetary biosphere that mimics Earth on another planet has yet to be verified. Mars is considered by many to be the most likely candidate for terraformation. Much study has gone into the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. However, a multitude of obstacles stand between the present and an active terraforming effort on Mars or any other world. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics and methodology of altering the environment of an extraterrestrial world.