Photosensitized oxidation is the key chemical process which causes cell toxicity in photodynamic therapy. The process is initiated by absorption of light by a sensitizer (Sens) to convert the sensitizer to an electronically excited state (Sens*). There are two principal reaction mechanisms, referred to as Type I or II, depending on whether the initial interaction of Sens* is with substrate or solvent, or with oxygen, respectively. The route actually adopted depends on oxygen and substrate concentration, and on substrate and sensitizer reactivity. Sensitizers may be dyes, pigments such as porphyrins, aromatic hydrocarbons, ketones or quinones, or a variety of other light-absorbing materials. Most types of photodynamic action involve absorption of visible light, although near-UV or infrared light can also be used in some cases.