Vespene gas is a basic resource used to produce advanced units for all races. It is green in color and emerges from the top of volcano-like structures called Vespene Geysers, like the one shown. The other basic resource, which is needed to produce all units of all races, is minerals.
According to Doran Routhe, a famous Terran scientist, solar systems outside of the Sun's could contain new resources. At least two have been discovered in the Koprulu Sector, Vespene and blue mineral crystals.
In order to transport and store Vespene gas, it must first be converted into a liquid form. Each race has a type of structure for performing this conversion: the Terran Refinery, the Zerg Extractor, and the Protoss Assimilator. Each source has a fixed amount of gas readily available, and it is collected 8 units at a time and brought to the base by a collector unit (Terran SCV, Zerg Drone, or Protoss Probe); only one collector unit can be inside the building collecting at any one time. Once the fixed amount of gas is expended, the source is said to be depleted, and then only 2 units of gas are collected at a time.
Generally speaking, the more advanced a unit is and the more abilities it has, the more Vespene gas it requires to produce. Flying units almost always require Vespene gas, as does any unit with an energy meter. Basic structures do not require Vespene gas to construct (except the Hydralisk Den), while advanced structures usually do. Terrans typically require Vespene gas to repair flying and other advanced units that were built with Vespene.
Because a player with no Vespene gas should be able to begin a mission, the collector units, the base structure ("town hall"), and the Vespene gas converter structure never require Vespene gas to construct. Each race also has a simple ground soldier that requires only minerals to produce, the Terran Marine, the Zerg Zergling, and the Protoss Zealot.
Vespene is widely sought after by the Terrans, Protoss and Zerg for different reasons. The Terrans use it as an energy source, in a manner similar to the way fossil fuels are used now. The Protoss use it as a catalyst for their psi-driven machines, and the Zerg use it for metabolic purposes and as a method of inducing evolutionary enhancements. It has been rumored that exposure to raw Vespene is responsible for the Ultralisk's improved biological carapace.
Vespene and mineral formations slowly regrow over time on planets such as Antiga Prime; this allowed the Terrans to survive in the Koprulu Sector.
It is unknown if Vespene gas has any relation to Terrazine gas, a potent psionic reagent, or to chlorine gas, which occurs in small amounts in volcanic gases along with hydrogen chloride.