The terraforming of Mars or the terraformation of Mars is a hypothetical procedure that would consist of a planetary engineering project or concurrent projects aspiring to transform Mars from a planet hostile to terrestrial life to one that could sustainably host humans and other lifeforms free of protection or mediation.

The terraforming of Venus or the terraformation of Venus is the hypothetical process of engineering the global environment of the planet Venus in order to make it suitable for human habitation.

The ethics of terraforming has constituted a philosophical debate within biology, ecology, and environmental ethics as to whether terraforming other worlds is an ethical endeavor.

Terraforming is well represented in contemporary literature, usually in the form of science fiction, as well as in popular culture..