While teeth usually feel smooth to the touch, on a microscopic level one can find surface irregularities, micro-cracks and defects - all which trap plaque and stain from dark-colored foods and beverages. As the stains accumulate, the tooth becomes darker in shade. These stains are extrinsic (on the surface) and are removed by cleaning the surface. At a routine hygiene visit, this surface-cleaning is the "polish" part of the appointment - usually after a hygienist finishes removing tartar from the teeth. Extrinsic stains build up at different rates on different people/different teeth. Smokers and tea-drinkers accumulate a lot of stain, and so do people who have lots of microscopic irregularity in their enamel. These people may opt to have their teeth polished by their hygienist more often than the regular twice-per-year appointment.

In addition to extrinsic stain, teeth have color inside (intrinsic color). At the surface of a tooth is the enamel covering. The enamel is translucent and allows show-through of the colored substance that makes up the next layer of the tooth. This layer is called dentin and makes up the majority of the structure of the tooth. If the dentin is a light white shade, it shows through and the teeth appear white. If the dentin is a dark yellow shade, the teeth appear dark yellow. It is the dentin that is the target of professional whitening procedures (bleaching). Whitening gel, made up of either hydrogen or carbamide peroxide, acts to lighten the intrinsic color of the dentin, thereby lightening the natural shade of the teeth.

As people age, enamel wears and becomes thinner, making the dentin more visible through the enamel. This is one reason that teeth appear more yellow as people age.

Some teeth appear very dark brown or gray. In these cases, the dentin has usually been affected by trauma, antibiotics, or exposure to unusually high fluoride concentrations. If these forces affect teeth very early in their growth/development stages, the teeth turn a very dark color. Until recently, professionals believed these teeth would not respond to bleaching methods. Studies now show that in some cases of very dark teeth, long-term bleaching will work to lighten these teeth.

Finally the presence of periodontal (gum) disease, tooth decay or disease may result in darker-looking teeth. For this reason, it is essential to consult with a dentist (and have a thorough exam) before beginning any whitening procedure. It is important to have a healthy mouth before whitening. Otherwise, disease or decay could spread resulting in loss of newly whitened teeth.

In summary, some of the main causes of staining or discoloration are: plaque, dark-colored foods and beverages, smoking, trauma, certain antibiotics taken during tooth development, extremely high fluoride concentrations in well water, and age. A combination of professional cleaning and a whitening/bleaching system will whiten discolored teeth.