Game Usage
 Zerg vs Terran
Zerglings are effective at all times against Terran. Massing Zerglings with early speed upgrade makes them effective if not scouted correctly by the Terran. In later stages of the game, they are often paired with Lurkers or Mutalisks. When Hive tech is reached, Zerglings become very powerful with their adrenal gland upgrade.
 Zerg vs Protoss
Protoss are also susceptible to early speed upgraded Zergling rushes, if not scouted correctly. Eventually, Zerglings are used with Lurkers against Zealot/Dragoon armies. After the adrenal glands upgrade is researched, Zerglings are incredibly effective when paired with Ultralisks.
 Zerg vs Zerg
Before Mutalisks come into play, Zerglings are the primary offensive unit in ZvZ. Since all ZvZs lead to Spire tech, players will often try to overrun their opponents with mass Zerglings before Mutalisks hit critical mass. It is ideal to keep up Zergling production with your opponent.