The Zerg are composed of several different types of creatures that have been integrated into the Swarm by the Overmind. These creatures, or breeds, have been selectively evolved to become efficient killers and to assist the Zerg in their quest for ultimate power. The Zerg do not utilize technology in the common sense, but their natural weaponry and armor is comparable to the most advanced gadgetry employed by any other species. This biological evolution, combined with an unmatched savagery and their blind devotion to the Overmind, make the Zerg an extremely formidable and deadly foe.

Maggot-like, Zerg Larvae contain within them the genetic code for all other Zerg breeds. When a new breed of Zerg is needed, an individual Larva will enter a pupal state and begin replicating the DNA of one specific Zerg breed at an astounding rate. After a short gestation period, the new mature Zerg hatches.

Like the Larvae from which they hatch, Zerg Drones contain within them the genetic code required to morph into any of the large Zerg "buildings", which are actually living (but immobile) structures. They can only grow on areas where the Creep has established itself, as they require its nourishment and building material. The Drones also harvest any raw materials that are needed. Vespene gas is very important to the Zerg: the high energy state of the gas powers their accelerated metabolisms and is essential for the production of the more advanced Zerg breeds.

The extent of the Overlords’ control over the Zerg is unknown, but it is clear that these flying behemoths provide direction for the other Zerg breeds. Although they resemble the Portuguese man-o-war of Earth, they are fairly docile creatures that tend to young larvae and transport other Zerg across the depths of space by carrying them within their shelled bodies.

The small, savage Zergling is one of the few breeds that a core species has been found for. It is so closely related to the dune-runners of Zz’gash that it is believed that the Zerg at some point encountered and "absorbed" the DNA structure of the dune-runners. Mutated by the Zerg to be more efficient killers, the voracious Zergling are capable of ripping a larger creature to shreds with their razor-edged forearms and sharp fangs. Their DNA structure is so simple that two Zerglings can be created from a single Larva.

The Hydralisk is easily the most versatile breed for the Zerg. With the ability to strike both ground and air targets, Burrow (once evolved), and health regeneration, the Hydralisk is a force to be reckoned with. Also with their short build times and relatively low cost, they can be produced in mass numbers to overwhelm the enemy

Frankly, the Mutalisk is an enigma. It is not known how they are capable of maneuvering through a vacuum, or even of controlled flight within an atmosphere, but they have been able to stand toe-to-toe (so to speak) against Wraith fighters along the Coreward border worlds. The Mutalisk's attack is quite unusual. The Mutalisk produces and projects much smaller, voracious creatures that rapidly fly between enemy targets as it disintegrates explosively.

The Scourge serve as kamikaze-attack flying units, capable of causing 110 damage to aerial targets. Like Zerglings, Scourge spawn in pairs from a single Larva and can be produced in mass numbers in a very short period of time.

The Zerg "Queen" (a misnomer, as it does not produce Larvae) is capable of producing biotoxins that have dramatic effects on both Zerg and other lifeforms. There have also been reports of Queens spawning smaller Zerg creatures (Broodlings) by injecting other species with thrown egg clusters. Like the Overlords, the Queens appear to tend to Larvae, and are generally found near the central nest.

One of the newer and deadlier Zerg Strains is the Lurker. These creatures serve as defensive warriors for the Hive Clusters and outlying Zerg colonies. Bred from the Hydralisk strain, Lurkers emit waves of supra-dense spines against their enemies. However, Lurkers must actually Burrow underground to release these subterranean attacks. Any warriors on the surface who are caught by these spines are instantly impaled. These spines themselves are capable of tearing into flesh, steel, and even reinforced armor plating. The only weakness of the Lurker is that it is virtually defenseless while above ground. Only in StarCraft: Brood War

This breed is very rarely encountered, and we can be thankful for that. Vaguely resembling a scorpion in appearance, Defilers are veritable cancer factories. They generate a number of virus-like biochemicals which are capable of overpowering even the most effective anti-toxins. Defilers also appear to feed on other Zerg for biomaterial to produce these chemicals, and are often found with Zergling escorts. This self-destructive behavior only underscores the Zerg’s single-mindedness.

The Zerg use these massive creatures as living tanks. Their immense mandibles have a monomolecular edge, enabling them to cut through armored vehicles and infantry alike. A handful of these creatures are believed to have been responsible for the death of an entire Confederate garrison on Timul IV. The best way to deal with these beasts is from the air.

Believed to be the nesting form of the Mutalisk, this is a secondary stage that has a much thicker armored carapace and a longer ranged plasma attack. The Guardian is, however, noticeably slower than the Mutalisk, and as such is used primarily to defend the vulnerable Zerg Larvae.

Bred from the Mutalisk strain, the Devourers are a deadly addition to the Swarm. These large, flying behemoths are capable of spewing their vile acid at any airborne ships or creatures, causing significant corrosive damage. The venom of the Devourer consists of toxins that can eat through any known substance, including the reinforced armor plating of Terran and Protoss capital ships. The venom also contains Acid Spores that hinder the fighting ability of the target, and these Spores often splash off the target to hit additional nearby enemy craft. Only in StarCraft: Brood War

When a Queen uses its Spawn Broodling ability, the target creature unwillingly fertilizes and serves as the breeding ground for a near instantaneous metabolization and birth of a pair of Broodlings. These Broodlings have a limited lifespan and are weak attackers, however, their mere presence should be a reason to rejoice since in their place an enemy host (hopefully a powerful one) had to die to give birth to them.

Zerg Queens have the ability to infest severely damaged Terran Command Centers, saturating them with parasitic bio-toxins. Once the Command Center is fully infested, its previous occupants can be turned into mindless suicide soldiers capable of mass destruction known as the Infested Terrans.

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