Tooth whitening is an esthetic procedure for tooth colour correction. Causes of tooth discoloration can be divided into internal and external. Tooth-bleaching techniques are based on hydrogen peroxide as an active agent. It is applied directly, or produced in a chemical reaction from sodium perborate or carbamide peroxide. Although the action mechanism of hydrogen peroxide is not fully understood, it is considered to be an oxidation reaction, where the pigmented molecules are broken down into the smaller compounds which diffuse out of the tooth. Since the tooth whitening is a common procedure, there are concerns about the biologic adverse effects of hydrogen peroxide. The accumulated data demonstrates that, when used properly, peroxide - based tooth whitening is safe and effective. The most commonly seen side effects are tooth sensitivity and gingival irritation, which are usually mild to moderate and transient. Cervical root resorption is a possible consequence of internal bleaching. Bleaching products do not cause major alterations of enamel and dentin structures or their surfaces. Also, there is no evidence of a toxic and cancerogenic risk to humans from hydrogen peroxide if used at exposure levels associated with tooth bleaching. For a successful therapy, it is essential to define indications and contraindications, to correctly set the diagnosis and to select the most suitable technique for teeth whitening.