How does FIFA determine number of countries from each region?

Each region gets a different number spots for the World Cup (e g., Europe 16, Africa 9). How is this determined? I am sure there is some controversy there as with anything FIFA related. I remember hearing that the success of teams within the region is a factor. Is this based on success in previous World Cups? Other tournaments? World rankings? Obviously there is a disparity across the regions when it comes to number and quality the of teams, I am curious how this is translated into the specific number that go from each region.