Arcade cabinet
In the earlier stages of the game, there are a few cabinets of the original Area 51 arcade game, complete with red and blue light-guns.

Defeating Grays
To defeat Grays that use shields, mutate and fire parasites at them. It will bring down the shields faster than any other weapon in the game.

Defeating the Super Beast
When fighting the Super Beast, rather than running straight at him, run around in a circle. When he gets near an explosive barrel, shoot it. It does more damage then the BBG. Watch out for his plasma shooting arm as well. When you finish, Edgar will talk to you, and you get to see him.

Infrared
If you start the game, only shoot the first three STAAR team members. If you do this right, you will have an infrared sight.

Secret Rooms
Level 2: Shoot all the highest windows in the far wall in the hangar. When you are in the truck, shoot all yellow barrels, and you will discover another secret room.

Level 4: When you are driving on the STAAR jeep, shoot every yellow box.

Level 5: Shoot the sign which shows the words General Weatherby on the second door right away when this door opens.

Level 7: Shoot all yellow barrels in the first area where the four purple zombies are throwing barrels.

Secret tapes
Find and scan all five databank items in a level to unlock the secret for that level. The final secret of the game is unlocked when all prior secrets have been found.

Spooky Hidden Rooms
After you have ride through the airfield, and you have got the ladder that hangs out of the chopper, look with attention to the paintings that hang at the wall. When you shoot the paintings and all the officedoors, you will arrive in a strange room.

The End
Good End: Shoot all the panels of the mothership
Bad End: If you let the mothership escape

Unlock Gray Alien in Multi-player mode
Successfully complete campaign mode under the Hard difficulty setting.

Unlock Harder difficulty
Successfully complete campaign mode under the Medium difficulty setting.