When player skill levels differ widely in a competitive First-Person Shooter (FPS) game, enjoyment suffers: weaker players become frustrated and stronger players become less engaged. Player balancing techniques attempt to assist the weaker player and make games more competitive, but these techniques have limitations for deployment when skill levels vary substantially. In this thesis, we developed new player balancing schemes to deal with a range of FPS skill difference, and tested these techniques in a series of five studies using a commercial-quality FPS game developed with the UDK engine. Our results showed that our balancing techniques (Combo and Delay) are extremely effective at balancing, even for players with large skill differences. These techniques also led to higher enjoyment of the game by players of all skill levels. Our studies are the first to show that player balancing can work well in realistic FPS games, providing developers with a way to increase the audience for this popular genre. In addition, our results demonstrate the idea that successful balancing is as much about the way the technique is applied as it is about the specific manipulation.