Mars is bitterly cold and dry, but robotic spacecraft have returned abundant data that indicate Mars once had a much warmer and wetter climate in the past. These data, the basis of the search for past or present life on Mars, suggest the possibility of returning Mars to its previous climate by global engineering techniques. Greenhouse gases, such as perfluorocarbons, appear to be the best method for warming Mars and increasing its atmospheric density so that liquid water becomes stable. The process of making Mars habitable for terrestrial organisms is called terraforming or planetary ecosynthesis. The process of introducing terrestrial ecosystems to Mars can be compared with a descent down a high mountain. Each drop in elevation results in a warmer, wetter climate and more diverse biological community. Beginning with a polar desert, the sequence of ecosystems passes through tundra, boreal forest, and temperate ecosystems where moisture determines the presence of desert, grassland, or forest. This model suggests a sequence for the introduction of ecosystems to Mars and the communities to search for potential colonizing species for Mars.