Terraforming is the process in which the terrain, climate and atmosphere of a planet is altered to suit the needs of those colonizing the planet. It normally involves inserting plants and/or flora, introducing new minerals or substances, or other changes to fulfill the required goals for the planet in question. It can take up to several centuries to fully terraform an Earth-sized planet, depending on the degree of terraforming that is needed. The UNSC uses terraforming processes to convert potential colonies into environments suitable for human habitation. The UNSC Odyssey was the first UNSC ship to carry terraforming gear out of the Sol system.
The UNSC uses devices known as "Atmospheric Processors" to push a planet's atmosphere towards a more tolerable composition, but human terraforming technology has its limits. Many planets are so inhospitable that they cannot be terraformed, which often results in the UNSC seeking already habitable planets. Understandably, these are extremely few and far between, which results in some colonies being extremely remote. Still, the UNSC has used several methods to terraform worlds, such as bombarding a planet with asteroids to lay a new mantle and crust over the planet's natural one. This process is known to trap helium-3 into the second, artificial mantle. The colony world Verge was known to have required this type of terraforming.
After the Human-Covenant War, humanity began to re-terraform their colony worlds glassed by the Covenant during the conflict, including Reach. While initial assessments for re-terraforming made by the Assembly in 2530 indicated that restoring a glassed world would take from 110 to over 300 years,  Reach had been at least partially restored by 2589, in only 37 years.
The Flood terraform their habitats through natural processes, converting the atmosphere and using local biomass to spread itself until it can form a Flood Hive. Beginning as small spores, these eventually grow into larger clumps of biomass, which convert the areas atmosphere to what is presumably optimal for Flood expansion, and have been known to infect large areas, ranging from areas of Installation 05, to the planetoid High Charity.
 List of appearances
- Halo: Reach
- Halo: Contact Harvest (Mentioned only)
- Halo: Evolutions - Essential Tales of the Halo Universe
- ^ Halo: Ghosts of Onyx
- ^ Halo: Contact Harvest, "chapter 1", page 32
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "Blunt Instruments", page 194
- ^ Halo: Reach, Data pad 14
- ^ Halo: Reach, campaign level, Lone Wolf
- ^ Halo: Combat Evolved, campaign level, The Library (343 Guilty Spark: "Your environment suit will serve you well when the Flood begin to alter the atmosphere - you are a good planner!")
- ^ Halo 2, campaign level, High Charity