There's an enemy alien, the Headcrab; in their default form they resemble a headless plucked chicken, with a huge mouth where a stomach would be. They latch onto their (mostly human) victims' heads, take over their still living bodies, and essentially turn them into zombies; once transformed, their bodies are split open from neck to waist, showing their internal organs and rib cage, as they scream with agonizingly deep, raspy, but squealing voices, as they slowly drag and lumber toward you, striking with their limbs. Another type hangs on the ceiling and resembles a huge mutated mouth, with large sharp jaws and teeth. It has a long, thin tongue that hangs from its mouth to the floor. When prey steps under it, it reels it in with its tongue, swallows it whole, with very bloody results. If you shoot and kill this enemy, it will turn inside out and regurgitate what it's eaten (including bones). The next type uses the same technique, but appears to take decomposed bodies as their hosts, resulting in them looking more like skeletons, with a small layer of muscle. Because of having less weight, they're extremely fast and rush at you, mercilessly tearing with their claws.

Early on, you receive the "Gravity Gun," and can use it to repulse or drag and hold objects with its gravitational force. In a later area, you encounter dozens of zombie Headcrabs. You can make use of several old and loose rotating saw blades and grab them with your Gravity Gun. If you shoot them with the saw blade equipped Gravity Gun, you will sever their bodies through the waist, with bloody and gory results. You also have the ability to grab compressed gas tanks with the gravity gun, and if used on zombies, they will explode in flames and scream in agony, before dying.

Your character will be killing dozens of enemies with a variety of weapons, including a crowbar, shotgun, rocket launcher, Gravity Gun, etc. You'll be fighting two seperate factions of enemies, the main being the "Combine," the corrupt "Civil Protection" that patrol the city, and the main villian's henchmen. They wear full body armor, and what appears to be a gas mask that covers their entire face. The mask must have some sort of voice box, because when they speak they sound like they're talking through a heavily distorted ham radio—they're a human/alien hybrid, genetically enhanced for their work. The Combine also use a device called the "Man-Hack," and it's about the size of a soft ball, and uses very sharp blades to manuever in the air, and knock into enemies, thus being known as the Man-Hack. The Headcrab are enemy both to you and Combine. The last enemy type are called "Antlions," and resemble an ant but are as large as lions, hence the name. They're a non-sentient species, and act like wild animals. You do gain the ability to control and command them with a device later in the game. But, beyond that, they have no obvious alliances.

Specific scenes of intense violence:

In the beginning of the game, you're on route to a train station that processes it's arrivals similar to cattle, giving no freewill. As you are being processed for your "release," they take you to a room that appears to be for interrogation, because there are large blood stains around a very uncomfortable looking chair. Once you escape processing, you enter an apartment complex soon to be raided by the Combine for no apparent reason. As they attack, capture and imprison the residents, you see a man consoling a woman, as she says she can't take anymore, and starts to cry as you leave the room. While you escape capture, several of the residents risk their lives to let you escape. These citizens help you throughout, as you're considered the only hope of their future liberty.
While escaping from the pursuing Combine, you come upon one area where a male and female character are being tormented by the Combine. The man is being beaten on the floor, as the woman is held back by another Combine. You save them, but as you leave not knowing if he is dead or just unconscious.
At one point you come upon a scene where two humans are on a rock sitting on sand. One of the men is heavily injured (with blood splattered on the floor around him). As you approach, the uninjured man warns you not to step on the sand. It soon becomes apparent why, as the ground shakes and Antlions burrow out of the sand, killing the injured man—if you don't intervene, both men die.
In a later area, a security camera feed shows a Combine body lying on a table, but it's not wearing any armor or a mask. You see a very pale slightly altered once human, with a voice box in its larynx; explaining the strange voice. You also see an unknown creature on another security feed, and it looks like an almost skeletal human, with a metal plate where its face would normally be; you also get a glimpses of this creature in the last part of the game.

Language: There are under a dozen uses of d*mn and hell, under a dozen uses of God, and one use of sh*t, bastard and a**. There's one partial use of the f-word, but it is drowned out by background noise, and if you have the subtitles on, the word is there, but it's censored. There is no option to turn language off.