Tooth whitening or tooth bleaching is the process of lightening the colour of human teeth. Whitening is often desirable when teeth become yellowed over time for a number of reasons, and can be achieved by changing the intrinsic or extrinsic colour of the tooth enamel. The chemical degradation of the chromogens within or on the tooth is termed as bleaching.

Hydrogen peroxide (H
₂O
₂) is the active ingredient most commonly used in whitening products and is delivered as either hydrogen peroxide or carbamide peroxide. Hydrogen peroxide is analogous to carbamide peroxide as it is released when the stable complex is in contact with water. When it diffuses into the tooth, hydrogen peroxide acts as an oxidising agent that breaks down to produce unstable free radicals. In the spaces between the inorganic salts in tooth enamel, these unstable free radicals attach to organic pigment molecules resulting in small, less heavily pigmented components. Reflecting less light, these smaller molecules create a "whitening effect". Peroxyacids are an alternative to hydrogen peroxide and also contribute to the breakdown of pigment molecules. There are different products available on the market to remove stains. For whitening treatment to be successful, dental professionals (dental hygienist or dentist) should correctly diagnose the type, intensity and location of the tooth discolouration. Time exposure and the concentration of the bleaching compound determines the tooth whitening endpoint.

The perception of tooth colour is multi-factorial. Reflection and absorption of light by the tooth can be influenced by a number of factors, including specular transmission of light through the tooth, specular reflection at the surface, diffuse light reflection at the surface, absorption and scattering of light within the dental tissues, enamel mineral content, enamel thickness, dentine colour, the human observer, the fatigue of the eye, the type of incident light, and the presence of extrinsic and intrinsic stains. Additionally, the perceived brightness of the tooth can change depending on the brightness and colour of the background.

The combination of intrinsic colour and the presence of extrinsic stains on the tooth surface influence the colour and thus the overall appearance of teeth. The scattering of light and absorption within enamel and dentine determine the intrinsic colour of teeth and because the enamel is relatively translucent, the dentinal properties can play a major role in determining the overall tooth colour. On the other hand, extrinsic stain and colour is the result of coloured regions that have formed within the acquired pellicle on the enamel surface and can be influenced by lifestyle behaviours or habits. For example, dietary intake of tannin-rich foods, poor tooth brushing technique, tobacco products, and exposure to iron salts and chlorhexidine can darken the colour of a tooth.

With increasing age, teeth tend to grow darker in shade. This can be attributed to secondary dentin formation and thinning of enamel due to tooth wear which contributes to a significant decrease in lightness and increase in yellowness. Tooth shade is not influenced by gender or race.

Tooth discolouration and staining is primarily due to two sources of stain: intrinsic and extrinsic (see Figure 2). In essence, tooth whitening primarily targets those intrinsic stains in which cannot be removed through mechanics such as a debridement (clean) or prophylaxis, in the dental office. Below explains in-depth the differences between the two sources of which contribute to such discolouration of the tooth's surface.

Extrinsic staining, is largely due to environmental factors including smoking, pigments in beverages and foods, antibiotics, and metals such as iron or copper. Coloured compounds from these sources are adsorbed into acquired dental pellicle or directly onto the surface of the tooth causing a stain to appear.

Extrinsic staining may be removed through various treatment methods: