The hypothetical idea of creating an Earthlike environment on another planet. Some scientists believe that astronauts could create an oxygenated atmosphere that would make Mars and other planets inhabitable.

http://www.users.globalnet.co.uk/~mfogg/ Good selection of essays and articles about terraforming the planets. The site has a gallery with space art depicting terraformed planets, as well as links to terraforming resources on the Internet and a comprehensive bibliography.