Alternate ending sequence
Successfully complete the last level of the game in campaign mode on the legendary difficulty setting. The new ending features a human and alien fighting hand to hand. The human will say "Its over", and then they hug because they know the explosion will kill them both. Also, the FMV sequence of each battle scene will be slightly longer.

Alternate dialogue
Playing on the legendary difficulty setting introduces new dialogue not heard in other modes. For example, after being massacred by a force of Grunts, Jackals and Elites, one of the grunts asks, "Can I have his helmet?". Also, your fellow Marines seem to be a bit more chatty (as well as profane). Also, on the legendary difficulty setting, view the entire first FMV sequence. During the sequence, the General says things very differently when compared to the sequence under the easy difficulty setting.

Final ending scene
Successfully complete the game on any difficulty setting and wait for the credits to end. The game will show the Monitor fly across the screen and laugh.

Instant full life
This trick works better in co-operative mode. If you need life and you have a partner, wait until you have killed 90% of the enemies around you. Then, have your partner hit you in the back with a gun. You will respawn with full life and assault rifle ammo. In single player mode, you can frag yourself but your weapons might fly across the screen and you will need to find them.

Extra ammunition
If you shoot your fellow Marines you can pick up their ammunition. Make sure to only kill a straggler unless you are prepared to take on all of the Marines. They will turn on you if you kill more than one of them.

Extra shotgun ammunition
Take a shotgun that has 12 shots and 0 clips. After you shoot it a few times, you should be able to reload manually. You can do this about ten to twenty times.

Easy kills
To kill someone, it is often easier to run him over rather than shooting him. If you shoot him, it will take a few hits. If you run him over, it will take only one hit.

Stronger melee attacks
Jump in the air and your melee attacks will be stronger while you are falling.

Blue shields
In some levels there are blue shields that you can hide behind. Opponents cannot see you there, but you can see them.

Jumping in a Warthog
When you jump off into a pit or something else in the Warthog, do not press the gas while in the air. You will go farther and when you land your gunner will not fall out.

Warthog fun
Go to any level or game in multi-player mode that will allow you to have vehicles. Have one player get in a Warthog and have the other do the same with another. Run into each other with the Warthogs and you will go flying in the air, doing flips.

Double jump
After you jump, do a crouch. You will stay in the air longer and go slightly higher. This is useful for reaching ledges that you can just barely reach with a normal jump.

Jump higher
Use the following trick to get up to higher places, such as good sniper locations. When you jump, crouch and melee at the same time. Also in co-op mode, jump on your teamate's head then jump again. You will jump much higher.

Flying high
Select multi-player split screen mode and create a game that has unlimited grenades and only rocket launchers. When you start, go to an outside level and look down as far as you can. Next, run backwards and throw a plasma grenade. It should stick on your foot. Then, shoot the rocket launcher as fast as you can. If done correctly, you will fly up with the plasma grenade exploding while in mid-air, sending you flying higher.

Flying team members
When a person gets into the Warthog (gunner's seat), throw a plasma grenade on his body. Move away and watch him fly into the air.

Flying bodies
Go to multi-player split screen mode and play a game with unlimited grenades with two or more players. Kill one of the players, then get everyone else to stand in "accurate" grenade throwing range. Have each player throw two grenades on the corpse; this must be timed very accurately. Throw them on the hands, knees, or anywhere else, then step away and watch the body fly.

Survive almost any drop with any vehicle
Immediately before you go off any high drop, send your vehicle into a 360 circle turn. This works best with the Warthog or Ghost. You can fall an incredible distance and receive no damage.

Mobile explosion
When playing two player cooperative or four player multi-player game with teams, have at least one plasma grenade and a Warthog. Have player two stand outside and ahead of the vehicle. Have player one will drive head on at the enemy (human or alien). Have player two throw a plasma grenade on the Warthog as the person is driving. Have player one will hop out, sending the Warthog barreling at the enemy. If timed correctly, you can get both run-over and explosion kills. Note: This also works with the Banshee and Ghost, but can only be done once, as it will destroy those two types of vehicles.

Flare
Have at least two plasma grenades when your partner cannot find you. Throw one in any direction and without moving as quickly as possible, throw a second one. The first one will explode, sending the second one flying into the air. The grenade will explode at a very high point, acting as a flare so your partner can see where you are located. Note: Make sure that you do not move in any direction while doing this, or the second grenade will not go into the air, only sideways if anywhere.

Traitor
On a level where you can find a group of five or more marines (such as Assault On The Control Room), kill three marines quickly. The remaining marines will call you a traitor (shouting things like "He's loco!") and will start firing at you. If you shoot them, sometimes they will ask you if they owe you money or tell you to clean the mud off your visor. Also, after being considered a traitor, if you want to regain the Marines' trust, find some Covenant and kill them. If you kill enough of them, the Motion Tracker will show Yellow-Green points where your "fellow" Marines are and they will not shoot you, though they may aim at you. Note: This was done in the Halo level.

Piggyback ride
Have someone hop onto someone else's shoulders and have the person on top crouch. It looks as if they are getting a piggyback ride.

Fireworks
If you have just won a game or would like to celebrate, take two plasma grenades and throw them on the same exact spot. The first grenade will explode, sending the second grenade into the air. After about one to two seconds, the grenade will explode blue.

Sneak up on radar in multi-player mode
You can sneak up on the radar by crouching down when running.

Quick kill in multi-player mode
When you are facing a partner in multi-player and you cannot find ammunition, sneak up behind him and hit him in the back for an instant kill.

In split screen multi-player mode, equip you pistol. When you see your opposition, place the aiming circle on their chest. After each shot, aim higher until you are aiming at their head. If done correctly, it only requires three shots to kill and will leave your friends wondering what hit them.

This is a very interesting way to kill a player in a car. This trick requires is a plasma grenade and a human pistol. If your opponent tries to run you over, dodge him by moving to the left of the car. Try to get into the side seat of the car. Once this is done, throw a plasma grenade down. Jump out of the car and run. The pistol is used for backup. This may take a few times, and a few deaths.

Defeating Covenants
When Covenant dropship is getting ready to unload a group of Covenants, throw a grenade into the door as it opens. The blast will kill several of the enemies since they are all in one area. There are places (especially in the first level) where a groups of Covenants are, waiting for you to barge in and start shooting. If you can, sneak to the aliens and throw a grenade into their midst. It is best when a plasma grenade sticks to a Grunt, because he will run behind the rest of his companions. One simple grenade can take out five Covenants.

Defeating Flood types
Do not try to take out any Flood types with a sniper rifle. It will simply go through their bodies without killing them. The weapon of choice is the shotgun. You can take them down with one shot. Remember, the closer they are, the faster they fall. On the small crawling Flood, use your assault rifle in short controlled bursts. Once one pops, it will also pop the ones nearby. Save shotgun and pistol ammunition for the working class Flood to drop them fast. It is also a good idea to hit them with your gun after they fall to make sure they are dead. On the fat Flood, use the shotgun to pop them, then clean up with rifle. When you first meet the Flood, they will outnumber you in groups. For the large Flood, use a shotgun, rocket launcher, or most importantly, a fragmentation grenade. Do not use a plasma grenade because they will run towards you and kill you. When you get attacked by the Flood, after you make them fall down the first time, shoot their head off while they are on the ground. They will not get back up.

Defeating Grunts
When the Grunts are sleeping you can beat them to death with the butt of your gun without waking up any of the other Grunts. You can usually kill a four to seven of them before an alien spots you. They start sleeping in level 5, Assault on the Control Room. Just remember not to shoot or they will wake up.

Defeating Hunters
Normally Hunters are the hardest Covenant to kill, due to their powerful armor and shields. However, even the strongest armor has weak spots. If you look at their necks or backs, you can see an orange spot. Aim for it. One shot with any gun will kill them if you hit them there. The best way to kill a hunter is to use a sniper rifle to zoom in on the orange spot. If you are being chased by a Hunter or an alien with a sword, run backwards while facing the alien. Jump when they try to slash you. Throw a plasma grenade at the alien and it will stick to him. Turn around and run in a zig-zag pattern to make sure he does not catch you in the explosion. If you are playing cooperative multi-player mode, have player two run in front of the Hunter until his back is turned to player one. While the Hunter is trying to turn player two into mush, have player one run up to the Hunter's back and melee attack the orange spot. The Hunter should die with one hit. You can kill the Hunters in a head-on assault. Get in close and use the pistol or shotgun. Aim for the head holding down the trigger. Your first shot(s) will knock back the head, exposing the unarmored neck. One shot in the neck and they will die.

Defeating Jackals
Use a photon weapon to destroy a Jackal's shield quickly. When you encounter a Jackal, run up and hit it with your weapon or get out a gun that uses bullets, not energy. When you are a far way away from Jackals and have a sniper, they will sit in the defense position and leave very little uncovered. You will notice that on the right or left is a small notch in the shield. Aim for it and the Jackal population will be severely decreased.

Satisfying goals and challenges

Kill a Hunter with a melee attack (let him rush sidestep, then attack).
Kill a Golod Swordman with a melee attack (hit him with a pistol burst from very close, then immediately attack).
Run over enemies with a Warhog.
Blast groups of enemies with a Scorpion Cannon.
Skip a major part of level 5.
Shoot down a Banshee with a rocket launcher.
Shoot down a Banshee from a Banshee.
Finish the game in legendary mode.
Playing through the entire game co-op in legendary mode.
Taking out a friend (not in a vehicle) in multi-player with a plasma grenade.

Tips for playing in legendary mode
Always use a good combination of weapons, and think about your surroundings and enemies when choosing weapons. The sniper rifle is great until you reach the Flood. A good combination is the shotgun (the human pistol before you reach the Flood), an automatic weapon, and grenades. If you do not prefer to follow this example, then remember to use a combination of short and long range weapons.

Conserve ammunition. Never waste ammunition, always gather as much ammunition as possible, reload your weapon during any break in the action, and switch off any plasma weapon for a better one whenever possible.

Take it slow. Do not burst in to take all of the fire. Also, use the scenery as a cover. If you can fit behind it, you can hide behind it.

Use the enemies themselves to your advantage. Remember that each enemy type has their own mannerisms and characteristics.

Take shortcuts when possible. Here are some examples. In The Silent Cartographer level head directly to the security override station. In the Assault On The Control Room level, after you get one of the early Banshees, in the area after you go under the tunnel you cannot take the tank through, the double bridge will be directly above you. Ride the Banshee to the bridge and skip a large portion of that level. Taking this shortcut means there will be no more enemies throughout the rest of the level. In the 343 Guilty Spark level, go to the secret area with the 2-D trees and follow the cliff through the structure that goes indoors. In The Maw level, after the dropship gets taken down, you will keep going. Continue until you see the first open door on your right. Enter there and take another right. This can shave a good minute and a half off the time it takes to get to the Longsword at the end of the level.