Normally, I don't use "cheating devices" in order to beat a game, but there are some Game Boy titles that I have struggled to beat.  Most notably, "Double Dragon 3:  The Arcade Game".  However, the codes are very easy to input and are immediately accessible if you want infinite lives, infinite time, super moon jump, or other moves.  If you want a complete list of the Game Boy codes available, you need to go to the Game Genie website.  But it is NOT guaranteed that every Game Boy title that was released will have codes.  (And please note, the Game Boy Game Genie WILL NOT work for Game Boy Color games -- you need the GameShark for that!)