Terraforming was the process of modifying the environment of a planet or moon to make it habitable for life; Most humanoid life that engage in terraforming modify the world to meet criteria for class M planets.
Terraforming begins with selecting a planet which has the right mass, gravity, rate of rotation, and balanced periods of day and night. Federation efforts at terraforming were coordinated by Terraform Command and ensured both that there is no life on the planet, and that it never will naturally occur.
Next, water must be made available for life in water basins. This could be done using hydraulic landscaping, sometimes with the additional step of filtering and re-oxygenating the subsurface water. If the water is hypersaline (incredibly high salt content) portions of it may be desalinated to meet terrestrial and agricultural needs. Then, microorganisms are introduced as the first constituents of the biosphere permitting it's atmosphere into self sustaining oxygen production. As the biosphere expands, higher order plants are introduced. One of the key metrics of the process being successful is monitoring the amount of vegetation on the planet.
Industrial replicators would allow for manufacture of large volumes of volatile compounds and elements critical for life (Carbon dioxide, Nitrogen, Ammonia, methane, etc). Terraformers accelerate the process of terraforming; by doing so it only would take around 30 years to build a habitable world. Given in nature it would take millions or billions of years; or not at all given the planet. (TNG: "Home Soil")
- Blue Horizon – terraformed by Professor Gideon Seyetik (DS9: "Second Sight")
- Browder IV – a project that was assisted by the starships USS Hood and USS Enterprise-D in 2366 (TNG: "Allegiance")
- Caldos colony – established in the 22nd century (TNG: "Sub Rosa")
- Delta 2 – a Class 4 moon terraformed with Prototaxites stellaviatori in order to replenish the USS Discovery's supply of spores. (DIS: "The War Without, The War Within")
- Doraf I – in 2368, the Enterprise-D was scheduled to undertake a terraforming mission here (TNG: "Unification I")
- Genesis Planet – created in 2285 from the Mutara Nebula, but destroyed as a result of the Genesis Device, which contained protomatter (Star Trek II: The Wrath of Khan; Star Trek III: The Search for Spock)
- Mars – as of 2155, conditions on Mars were sufficiently altered to allow Humans to survive on the surface by only having to use small breathing gear and thermal insulation (ENT: "Demons")
- New Halana – terraformed by Prof. Gideon Seyetik (DS9: "Second Sight")
- Penthara IV – terraformed in 2368 to counter an ecological disaster (TNG: "A Matter of Time")
- Regula – a Class D planetoid whose interior was terraformed as the second phase of Project Genesis (Star Trek II: The Wrath of Khan)
- Velara III – site of the Velara Base; mission abandoned in 2364 after life was discovered on the planet (TNG: "Home Soil")
- Venus – site of several terraforming stations by the 24th century (DS9: "Past Tense, Part I")
- Weytahn – a Class D planetoid terraformed by the Andorians for habitation in the 2050s (ENT: "Cease Fire")