What is the cause of brown teeth?

Many factors: Brown teeth can be the result of many factors, including your diet, too much Fluoride in your water source when you were younger, and decay. If the color seems to be on the outside of the teeth, it could be from drinking tea or coffee, smoking and other dark foods and should be easily bleached out. However, darkness inside, or "intrinsic", can be from the dentin color and isn't easily lightened.