Headcrab
Headcrabs are your basic alien enemy. They're weak, but also very small, making them hard to hit. Headcrabs are usually found hiding in damp or dark areas, waiting for prey to ambush. When attacking, they launch themselves at their intended victim's head, making a screeching sound. While this would cause trouble for most other humans, Gordon's HEV suit protects his head.
Zombie
Zombies are humans that have fallen victim to headcrabs. The infestation has mutated their hands into long, sharp claws, and there is a giant gaping hole in their torsos. Zombies are usually found standing around doing nothing, but are sometimes found on the floor, feigning death until they are shot or someone approaches. To attack, zombies slash with their claws.
Hazardous Environment Combat Unit
Assassins
Alien Slave (Vortigaunt)
Alien Grunt
Alien Controller
Gargantua
Houndeye
Bullsquid
Barnacle
Itchyosaur
Gonarch
Nihilanth
The boss of the game.