Singlet oxygen, generated photochemically or chemically, has damaging effects on biomolecules and exerts genotoxic, virucidal and cytotoxic effects. This is of relevance for biological systems because singlet oxygen can be produced photochemically as a result of the irradiation of endogenous or exogenously applied photosensitizers with visible or ultraviolet light, or in dark reactions, e.g. by stimulated phagocytes during the so-called oxidative burst. In addition, there is increasing evidence that singlet oxygen has pronounced effects on cellular signaling events leading to the induced expression of a variety of proteins. A novel observation is the activation of transcription factor AP-2 and cellular signaling cascades comprising the activation of c-Jun-N-terminal kinases (JNK/SAPK) and the NF-kappaB system.