Half-Life

Invincibility:
Press Left, Square, Up, Triangle, Right, Circle, Down, X at the cheat screen.
Harry Owen.

Unlimited ammunition:
Press Down, X, Left, Circle, Down, X, Left, Circle at the cheat screen. Note: This does not affect throwable weapons such as hand grenades and remote bombs.
Harry Owen.

Slow motion mode:
Press Right, Square, Up, Triangle, Right, Square, Up, Triangle at the cheat screen.
Harry Owen and Dirk Sauer.

Invisibility:
Press Left, Square, Right, Circle, Left, Square, Right, Circle at the cheat screen.
Harry Owen.

Alien mode:
Press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle at the cheat screen.
Harry Owen.

Alternately, complete all nine Decay levels with an "A" rank. You will now be able to access the secret tenth mission where you will play as a Vortigaunt for the first time. Note: The Vortigaunt only has two attacks, the E-bolt attack and the claws. Note: You must start a new game or else the code will not work. After you start a new game, you will experience the game through the eye's of a Vortigaunt (alien).
Dakota Hebert.

Xen gravity:
Press Up, Triangle, Down, X, Up, Triangle, Down, X at the cheat screen.
Harry Owen.

Bonus levels:
Press Triangle, Square, Left, Right, Circle, X, Up, Down at the cheat screen. Choose the "Extras" at the options screen to load new levels found from other discs, such as the Official US PlayStation Magazine Issue 57 (June 2002) demo disc.

To get a bonus level in Decay, get one A in every section. The final level where you play as a Vortigaunt alien will then become available.
Pat1986.

All Decay missions:
Press R3 on controller two; L3, Up(2) on controller one; Down(2) on controller two; Left, Right on controller one; Left, Right on controller two; X, Circle, Start on controller one.

Hint: Starting tips:
After the "accident", you will discover a scientist giving CPR to a security guard. Wait 15 to 20 seconds to see the outcome, which usually results in the security guard being revived. He is not useful at this time, but return to him after getting the crowbar. Kill him with the crowbar so he can not fire back to get his gun and ammunition.

Soon after this, you will ride an elevator upward. Have the gun ready. Shoot the monster attacking the security guard; a couple of shots should do it. This security guard can cover you until you reach the main entrance area.

One important tactic in the early game is to use your "allies" to your advantage. When you no longer require the services of a security guard, kill him and take his ammunition. A scientist is also useful in drawing enemy attack. Be careful not to harm an ally when another security guard is present, as he will attack you. Also be aware that monsters can sense your allies; and even if you crawl, monsters can hear your allies' walk.

The beginning of the game also has the following glitch. There is a scientist sitting in front of a damaged instrument panel, in the area of the first scientist who opened a door for you. This scientist is invincible. You can shoot or beat him with a crowbar, and all he will do is whine. This also saturates the area with a large amount of blood.

Hint: Extra health:
Buy a soda from the random soda machines. If you are impatient, shoot the soda machine.
SteffieB14.

Hint: Defeating Gargantuas:
Gargantuas are also invincible. Anything you fire at them apart from explosives just bounces off them, and only makes them angry. Just dodge them and use your mind to defeat them. Sometimes you do not need to kill one in the Interloper level.
Dbgarrott.

Hint: Defeating Gonarch:
The Long Jump module can be used for more than moving between floating platforms. You can get extra height when you jump into a wall or other object. When you first get dumped in Gonarch's lair, to your right you may see white "plants" coming out of the ground. You can use these as stepping stones to get up on the raised area in the middle. This may not easy, but if you get up there Gonarch will have trouble hitting you with any attacks. It is best to use weapons like the crossbow for this part. You can also use the long jump technique to get to other places you are not supposed to, and it is useful for getting past annoying opponents quickly.
Solidus Snake.

Hint: Defeating Tentacle Aliens:
Tentacle Aliens are very difficult to deal with. They only attack by hearing. If you fire something loud, the tentacles will find and kill you in two hits. Use grenades to distract them in the blast pit. Another method is sound and movement skills. If you do this, the tentacles will not find you as easily when you are in their respective areas.

Hint: Office Complex mission: Water room:
There is a room full of water. You must deactivate a switch that gives energy to the room because the water is full of electricity. When you deactivate it the electricity will disappear.
Frank.

Hint: Saving ammunition:
Do not waste precious pistol ammo on headcrabs. Use your crowbar, as it is more useful.

Sometimes the best course of action is to not defeat enemies. You cannot always attack them with a crowbar, but it is best not to waste a few weapons you have just collected. You might need that ammunition later. This is true when you are dealing with Gargantuas and Tentacle Monster. Both of those aliens are invincible to bullets. When you get captured by the enemy soldiers in the Apprehension level mission 8, do not waste your ammunition.
Dbgarrott.

Hint: Unbreakable windows:
There are some windows that you cannot break with guns. However, you can break them with your crowbar.
Frank.

Hint: Using the pistol:
Use the following trick to save ammo for all guns other than pistol. Most creatures or persons can be killed with five or less direct shots of the pistol.

Crabs: 2 shots or 1 crowbar hit
Zombies: 4 shots
Electric Aliens: 3 shots
Trip Bomb: 1 shot
Machinegun on stand: 5 shots
Soldiers: 5 shots
Dogs: 3 shots
Spitters: 4 shots

Hint: Using guards:
If you have a guard, get him close enough and he will take care of almost anything for you. The only time you ever need to fire is if the creature is close to the guard.

Hint: Scoreboard:
Press Triangle in a Decay Game to display a scoreboard for both your characters. It will tell you how much damage you took, the amount of kills you earned, how many times your characters died in the missions, and your player's name.
Dbgarrott.

Hint: Re-center aim:
Tap R3 and the screen will become centered again.
Brian.

Hint: Barney gets cut in half:
When you enter the room where the lasers are shooting out the walls, run through. There should be a Barney (security officer) on the floor. Watch carefully and you can see the laser go across and literally cut him in half.

Glitch: Floating doctor in wheelchair:
Note: This trick requires a second player. When playing a Decay game with another player, get to the Rift level. When the handicapped doctor tells you to follow him and opens the door, do as he says. He will get in the elevator. Block his path so he cannot get in the elevator, but the door is open. Next, back up slightly and let him enter. Get behind him and run forwards while the other player pushes you from behind. The doctor should start to go down, and the door has closed on you, but it is still open. Jump frequently and press Forward quickly. If you jump onto the safety bar where the top of the other door is, he will get to the bottom of the elevator. If you stand on the bar, and he gets to the bottom, then he will launch back up and will float in the air. You cannot kill him with a regular gun from below. You must shoot him with the bazooka or from above to kill him.
YoshimitsuNik11.

Glitch: Man in blue:
Enable the "Alien mode" code and start a new game. Go through the hall past the main security room and go straight instead of right. A short way down that hall is the man wearing the blue suit. Use your electric attack to break the window and jump in. If you try to attack the man with the blue suit, it will not work.
Peanuts363.

Glitch: Dead guard talks:
After the experiment goes awry, go get the crowbar and come back to the place where the scientist is performing CPR on the dead guard. If you smack the guard with your crowbar, he will talk.
Peanuts363.

Approach a guard or scientist that tells you information about something. Use a gun that will kill them in one shot, such as the caliber pistol or the crossbow. Shoot them just as you approach them, immediately before they start talking. If done correctly, the guard or scientist will fall to the ground dead, but will still tell you what they originally intended to say.
Chris Whitney.

Glitch: Look through dead scientist's eyes:
When you walk around, you will at some point encounter dead scientists, sitting with their back towards a wall. Crouch and then crawl toward the dead scientist. You will glide through him. Continue to walk until you reach the wall, then turn around. You can now see the way the developers have made the skins for eyes and mouth and everything else. You can actually look through the scientist eyes.!
SeS-1.

Glitch: Partial invincibility:
When in decay mode and are playing both people, the second person is not invincible, even if you enable the "Invincibility" code.
SteffieB14.

Glitch: Sniper target:
Get to the part in the Surface Tension level where you are outside the building with all of the tripbombs and missiles overhead. Kill the aliens dropped by the alien ship. Then, continue to the right and go through the "alley", then go left to see a man on the ground. Note: Do not kill any snipers. Continue straight and over the barbed wire to the left. Go up the fallen tower and jump over both vent shafts. To the left, you can see a broken building with a flat surface , Jump to it. On your right will be a broken wall. Jump and climb up it. Go to the right until you see a small crack on the roof that you are standing on. Go down it carefully. Turn on your flashlight and look to your right. You should see the sniper as a target with a gun.
MAKCHAMP333.

Glitch: Extra lasers:
In the Surface Tension level, when you get to the storm drain hatch switch go to the tent to your right. Send one or two grenades behind the green boxes to blow up the Laser Trip Mines. Then, go back there and you should see a laser with no trip mine on it.
Stephen Pendley.