Half-Life's aliens come in all shapes and sizes, but they do have one thing in common: they aren't stupid. They'll perform sophisticated threat assessments, recruit others of their kind, and figure out ways to flush you from your cover. Sometimes they'll even run away. Hey, just because they're aliens doesn't mean they're suicidal.

Lucky for you, not everyone... or everything... in Half-Life is out to get you. Know what you're dealing with and don't go into battle unprepared.

Headcrab

The lowly headcrab is the first enemy you'll encounter, and it's by far the most common. These guys love to dwell in dark corners and other out-of-sight areas, trying to surprise victims. The sharp limbs of the headcrab can easily tear through your hazard suit, if you're not careful. Never take headcrabs for granted. They're the most hated, most annoying, and least understood of all xenofauna. Headcrabs move clumsily but quickly. Its leap - if not dodged - is not precisely fatal to its prey, for once the headcrab has attached itself to the skull of a human host, there commences a swift and horrible process of "zombification" which gives the host a continued existence of the most objectionable sort. If you or a companion are ever unavoidably headcrabbed, you would be well advised to put a bullet through the affected brain as soon as possible, for only massive and irreversible damage to cerebral tissue has any apparent effect on the headcrab's ability to "drive" its victim.

Zombie

Want to know what happens after you've been headcrabbed? Take a look. It sure isn't pretty, but don't gawk too long: these guys would just love rip you open and play jumprope with your intestines. Although slow and dimwitted (as zombies tend to be), their long, sharp, claws are deadly. Put zombies out of their misery by attacking them between swipes.

Alien Slave

While many of the alien species collected and studied by the Department of Xenotheric Husbandry exhibit a lifestyle that might be characterized as "undesirable" in human terms, the honors for most wretched existence must certainly be granted to the slave-class of alien, xenotherium subservilia. These pathetic creatures, bred for submissive behavior and unswerving loyalty, will attack and kill without mercy when so ordered by their superiors. In such instances they will wage war with an insect-like vigor, heedless of personal risk, fearing the wrath of their masters more than any possible harm an enemy may inflict. On the other hand, in the rare situations where the alien slaves are out of the direct protection and supervision of an overlord, they will flee without shame and may be considered harmless. The slave species is hardy, impervious to discomfort, and due to the requirements of its breeders, undoubtedly possessed of extremely low intelligence. It is all the more remarkable, therefore, given a breed incapable of conceiving the idea of revolt, that the alien overlords find it necessary to burden their slaves with a metallic torque or slave-collar, which torments and eventually executes any slave that unwisely attempts to remove its yoke. Beware of the slaves' slow-charging energy attack, which is powerful enough to fry you extra crispy with a single blast.

Barnacle

Named for Dr. Louis Donaldson, its discoverer and first victim, the air-barnacle (through a freak of convergent evolution) resembles an unnaturally large member of the terrestrial subclass of Cirripedia, which includes the common goose-neck barnacle. But while the mundane barnacle passively filters nutrients from shifting ocean waters, the xenomorphic dry-land variety takes an extremely active role in the capture of any lifeforms unfortunate enough to enter its habitat. They have been occasionally mistaken for stalactites and light fixtures, and the investigator who values his life should always perform a thorough advance survey of ceilings and before entering caverns and corridors. If you are unfortunate enough to get caught by a barnacle, shoot at its mouth to free yourself. React quickly, or you're food.

Houndeye

The common "houndeye" or "sound dog" is a pack-animal par excellence. A houndeye apart from its pack is an unhappy and vulnerable creature, but in groups of three or more they will exhibit resonant behavior, emitting destructive harmonics capable of rupturing the internal organs of their prey. Deafness and migraine headaches are only two of the symptoms reported by survivors of houndeye attacks. However, if well-fed, they will not engage in harmonic behavior except when threatened and during their seasonal mating spree.

Bullsquid

The natural range and habitat of this supreme predator remain uncertain. While bearing signs of aquatic specialization, the bullsquid's powerful limbs and characteristic neurotoxic (and reportedly psychoactive) "venom-squirt" suggest it has adapted to a terrestrial environment. It is possessed not only of a ferocious appetite (necessary to support its immense bulk) but also of an overwhelming libido. In short, the bullsquid will attempt to eat or mate with almost anything that crosses its path, without regard for species or any apparent reproductive necessity, since dissection clearly indicate that every gastropolypus is hermaphroditic. Additionally, it has been speculated that to fauna of its native environment, the bullsquid's phlegm might have aphrodisiac qualities; but in terrestrial mammalian creatures, it is uniformly lethal.

Alien Grunt

Footsoldier of the invading army, this alien myrmidon relies on telepathy for coordination with its troops, but is no less dangerous when encountered in isolation. Like many inhabitants of the Portal Dimension, it appears to be a colonial creature, harboring several varieties of organic parasites which complement its destructive nature. Most dangerous are swarms of "thornets," heat-seeking autonomous "insects" which breed in their host's paracoelomic cavity and are only too happy to attack and consume the first thing they see upon release. The gleaming partial body armor at first appeared to be the product of an advanced weapons technology; but closer examination of the occasional moribund specimen has shown it to be a biologically extruded exoskeleton, synthesized from a diet rich in certain xenotic minerals. This suggests that long separation from the Portal Dimension, and a diet deficient in alien ores, might lead to the eventual weakening and demise of the alien troops. It is not, however, recommended that Earth rely on poor nutrition to repel the invaders.

Tentacle

Exhibiting characteristics of both the plant and animal kingdoms, it is speculated that the immense specimens reported for this highly sensitive xenosuctorian are a function of the Terran atmosphere. Lacking eyes, ears or other animalian sensory organs, h. sensitiva is nevertheless capable of detecting and targeting emitters of motion and sound in its immediate vicinity, by some process which is incompletely understood. The Tentacle, like some orchids and bromeliads, is most likely an epiphyte, deriving nutrients from the air and detritus; which theory is supported by the observation that h. sensitiva does not appear to eat what it kills, but conceals its prey beneath its root mass and absorbs the byproducts of decay. Stay quite in the presence of tentacles, and/or distract them if you want to stay alive.

Ichthyosaur

With this thing roaming around in the water, nothing is safe. A ferocious predator, the Ichthyosaur can ravage even the most hardy prey in seconds. Don't let its large size fool you; the Ichthyosaur can easily move through the water at great speeds.

Snark

Snarks are very simple creatures with limited capabilites. But if a snark is allowed to get close, they can cause quite a bit of damage, especially in groups. Snarks can be somewhat tamed and used as weapons, but due to their limited mental capacity they'll attack whatever they can find. Including their "master."

Garg

This blue badass can take insane amounts of punishment. Don't bother trying to take this guy out using conventional methods, you'll only waste your ammo. Oh, and don't get too close. Garg doesn't like people invading his space. Don't make Garg angry.

Alien Controller

This flying freak doesn't show up until later in the game, which is a good thing. They attack with energy blasts and are very hard to kill due to their constant movement.

Big Momma (AKA Gonarch)

Ever wonder where baby headcrabs come from? Now you know. Don't mess with Big Momma; if she doesn't crush you to death she'll send out her babies to lend a hand. This is one bad mother.

Baby Headcrabs

Isn't that cute? Baby headcrabs are much like regular headcrabs, except that their smaller size makes them much harder to kill. In other words, they're even more annoying.

Grunts

These guys are here to clean up that little mess you made, and they aren't exactly happy to see you. Grunts are usually armed with a 9mm Assault Rifle and a healthy supply of grenades, but you'll encounter some packing shotguns, rocket launchers, or operating turrents. These guys aren't kamikaze morons; they're experienced troops. They'll work together to take you down. Listen to their radio chatter carefully, as their communications may contain clues.

Assassins

Like women in skintight leather? Well guess what: they don't like you. You don't want to mess with these government trained killers. Assassins possess great speed, agility, marksmanship, and stealth. One of the toughest enemies you'll encounter, and by far the most intelligent. These ladies know that the quickest way to a man's heart is through his sternum.

The Man in the Suit

There is no intelligence available for this individual. His motive, role, and identity are unclear at present.

Next: Weapons