The Zergling is the earliest attack unit the Zerg player has access to. Zerglings can be morphed after construction of the Spawning Pool. They are melee units with low health individually but incredible damage for the cost. They spawn in pairs from a single Larvae at the same cost and supply as Marines but for two units.
Zerglings are vastly augmented by two upgrades. With their Metabolic Boost upgrade at the Spawning Pool they have one of the fastest movement speeds, second only to Vultures with the Ion Thruster upgrade. Adrenal Glands available at the Spawning Pool with Hive-tech improves their attack speed considerably. Zerglings can further be upgraded at the Evolution Chamber to improve both attack and armor.
Metabolic Boost, or Zergling Speed, increases the movement speed of the Zergling which allows them to be harassment units and overall increases their strength in an army. Zerglings with this upgrade are commonly called Speedlings.
Adrenal Glands, which requires Hive-tech and is an essential upgrade to most late-game Zerg Strategies, increases the attack speed of Zerglings. Zerglings with this upgrade are commonly called Cracklings (or Adrenalings, to a lesser extent).
Zerglings are quite powerful in the early game, especially when they have the speed upgrade, as they are able to choose their fights and engage any ground unit, but they are hampered by their low health. They become less useful in the mid game against ranged units that can attack all at once and kill them off before they are able to reach the opponent. Nonetheless, Zerglings are still built in many strategies to either absorb damage or perform flanks, as well as to comprise surrounding attacks. The Adrenal Glands upgrade, combined with Melee Weapons upgrades, makes them very powerful in the late game due to their high damage output, even though they still have a quite low number of hit points for a late game unit. These upgrades allow a small pack of Zerglings to quickly dispatch enemy buildings.
The Melee Weapons +1 upgrade is extremely powerful since Zerglings make so many small attacks, although it can likewise be negated by the enemy's armor. Since so many are made and a loss of a few Zerglings is not vital to the Zerg, they can often be burrowed around the map to provide sight in scouting opponent movement.
In Zerg mirror match-ups, the early game will frequently feature some Zergling exchanges. The Zerg player must actively scout to ensure an adequate number of Zerglings are morphed to counter that produced by the opponent. A small advantage or disadvantage in the number of Zerglings can decide the game at this point in the game, so players will often try to have at least as many Zerglings as the opponent.
Focus firing comes into play here and will often turn the tide in the favor of a player. Typically, players use Zerglings in the early game to pick off enemy drones, especially once Zergling speed is researched. Zerglings reach peak effectiveness immediately before large enough numbers of Mutalisks hatch as they are the only attack unit available to the Zerg. Zerg players sometimes mass Zerglings and attack immediately before the opponent's Mutalisks hatch. Afterwards, a large number of Zerglings can finish off and destroy the opponent's base since three Mutalisks cannot kill Zerglings effectively and more Zerglings can be rallied to the opponent's base. In Zerg vs. Zerg, only the Zergling speed upgrade is researched since most games do not last beyond Lair tech.
Zerglings are an important unit against Terran, and they are used in all stages of the game. They are weak and easily killed by themselves against Terran speed and range, but when paired up with higher tech units like Lurkers, Mutalisks, and Ultralisks, they become an indispensable unit to the Zerg army.
In the early game, Zerglings are used to kill scouting SCVs and vulnerable Marines, as well as to scout. Sufficient numbers of Zerglings may make the Terran retreat as it builds defensive Bunkers or Wall in. Zerglings can also be used for all-in rushes, such as 3 Hatch Zergling.
In the mid game, Zerglings are used to possibly damage Missile Turrets or to reduce Marine numbers for the incoming Mutalisk Harass. Alternately, in conjunction with Mutalisks, Zerglings are used to make a large Mutaling army to kill or block the Terran's mid game push. Sometimes, Zerglings are paired up with Lurkers and are used to surround (and damage) the enemy Terran army and prevent the Marines from spread apart to dodge Lurker spines.
In the late game, Zerglings become the primary attackers because of the Adrenal Glands upgrade, and their usefulness is further increased when paired with the Defiler unit. Generally the high firepower of a massed Terran late-game infantry army will slaughter almost all Zerglings that approach it, but under a Dark Swarm, Zerglings can safely approach and deal their high melee damage. When Plague is cast on the Terran army, Zerglings can rip it apart because of their large numbers and high maneuverability. The final evolution of the Zerg army incorporates Ultralisks with Zerglings (and possibly Defilers), increasing the damage output of Zerglings now that Ultralisks draw fire from Terran infantry army.
When fighting Protoss, most Zergs will morph four to six Zerglings in the early game in order to do deny the scouting Probe and, depending on the Protoss' build, to force Cannons at the expansion if the Protoss fast expanded. Otherwise, if a Protoss uses a 2 Gate Zealot or some rush strategy, they are used to defend the expansion base until Sunken Colonies are finished, and possibly later.
Against a Protoss FE build, it's also common to research Zergling Speed upgrade to totally deny Probe scouting, and possibly going for an all-in Zergling attack. Earlier Zerglings (Overpool/9 Pool) means that a FE Protoss has to invest in defense earlier, so that slows down the Protoss player's tech, while more economically oriented builds (12 Pool, 12 Hatch and all the variations after it) have delayed Zerglings, but give you a stronger economy.
In the mid game, the Protoss will generally have an advantage in upgrades over the Zerg player. The Zerg player is generally one down in the Carapace upgrade vs the Protoss Attack upgrade, which means that Zealots can kill Zerglings in two hits rather than three. Therefore, Zerglings are rarely seen midgame, and usually replaced by Hydralisks and Lurkers.
Oftentimes Zerglings are sent to scout for expansions (and take them down before the Cannons warp) and Protoss army movement.
- Zerglings are usually paired mid-game with Lurkers, as in the 5 hatch Lurker/Ling mid-game strategy to repel Zealots and Dragoons from the Lurkers.
- The Sauron Zerg style also incorporates large numbers of dual-upgraded Zerglings and Hydralisks, but is generally weak against modern Protoss builds, unless an early game advantage is gained.
- In a counter to Corsair/Reaver or Corsair/Dark Templar build orders, Burrow is researched and Zerglings can be burrowed all around the map, essentially providing the Zerg player map vision of the key locations. This enables the Zerg to keep track of any Protoss movement.
In the late game, Zerglings will usually have the Adrenal Glands researched, which gives them a significantly higher damage output. Additionally, if they are paired with Ultralisks, and possibly with Dark Swarm for support, they have a higher survivability chance. Thus, this enables them to deal a high amount of damage in a short period of time.
A small group of Cracklings (and Dark Swarm) can destroy an Expansion or Gateways in seconds, so late-game Zergling attacks or drops (respectively) are very viable for decimating such bases. It is also very common to draw the Protoss army on one side of the map just to send a group of Zerglings to take down an Expansion on the opposite side.