Type oh dear (case-sensitive) at the main menu to enable single player cheat mode in the North American version of the game. Activate the following cheats by entering the corresponding codes. Note Type levelx followed by SPACE for the European version of the game:

CHEAT FUNCTION - CHEAT CODE
Full ammunition - ALT + 1
Weapon upgrade - ALT + 2
Full energy - ALT + 3
God mode - ALT + 4
Wow mode - ALT + 5
End mission - ALT + 6
Deactivate enemies - ALT + 7
Helicopter mode - ALT + 8
Tank mode - ALT + 9
Speed mode - ALT + 0
Teleport to safety - ALT + CTRL + 0

Type emertxe at the main menu to activate Assault mode. Activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
Fly a UFO - ALT + U
Unknown - ALT + F
Unknown - ALT + L
Unknown - ALT + T

Enter HAMMER as a name in network mode to have a set of bonus pre-recorded messages become available during gameplay.