Cheers, I found an explanation which helps me understand it better. As this is thrown around so much these days I thought I would have a go at explaining it What goes on when you play a game. Simplified - The CPU sends draw calls to the graphics driver, These draw calls could be anything from rendering a little tree to a mega battle with loads of explosions. - The graphics driver then turns the draw calls into a scheduled rendering list and feeds it to the graphics card. - The graphics card renders the list sent to it by the graphics driver. My quite rubbish explanation Now as we all know CPU's have varying amounts of power, And we also know that games don't just consist of graphics. There's sound, Physics, artificial intelligence and general game management code all running and requiring processing at the same time that these graphics draw calls are being processed. Now if a large part of a CPU's resources are being consumed by sound, physics or artificial intelligence then there's not a lot of spare CPU processing cycles left over for processing the draw calls. Now if you have a relatively weak graphics card this isn't a problem as the CPU could easily keep the graphics driver and thus, The graphics card fed with rendering information. Now throw in a high end graphics card and you're in trouble, With the CPU using it's processing power on other aspects of the game the monster GPU is sat idle waiting for rendering information. And that is a CPU bottleneck. It's also the reason why games benefit from quad core support as there's more cores and CPU cycles to generate these draw calls while other cores handle other aspects of the game. How to tell if you're CPU bottlenecked : - Running at higher resolutions doesn't affect your frame rate. - Low GPU usage - High GPU usage with low on screen action but usage drops when loads of enemy's/physics work loads are being processed on screen. - Adding a second or third graphics card has provided no change in frame rates What to do if you're CPU bottlenecked : - Overclock your CPU - Upgrade CPU - Turn off as many back ground programs as possible to try and free up some CPU cycles - Turn up graphics settings to try and move the bottleneck over to the graphics card by giving it more work to do. - Turn up anti-aliasing ( Works best for me ) I'm no expert but hope it helps a few people to understand How does a CPU bottleneck a graphics card.