Most causes of tooth discolouration fall into two main categories: extrinsic and intrinsic stains.  Extrinsic stains are those that affect the surface of the enamel, which is the outermost layer of a tooth. These stains can typically be removed or corrected through professional dental scaling and polishing and/or dental whitening. Intrinsic stains however are those that affect the underlying tooth structure know as dentine. These stains are generally more difficult to remove. There are several reasons teeth become discoloured however lifestyle plays a big part in the yellowing process.  Individuals who smoke or chew tobacco, drink coffee and/or tea are generally more susceptible to discolouration. However, genetics, environment, age and even some medications can have an impact on the whiteness of your smile.