UVA irradiation (320-400 nm) comprises about 95 percent of incident midday solar ultraviolet irradiation. It penetrates skin much deeper than UVB irradiation. The absorption of UVA irradiation in endogenous chromophores frequently leads to the generation of reactive oxygen species such as singlet oxygen (O-1(2)). O-1(2) is an important biochemical intermediate in multiple biological processes. Beside other procedures, the direct detection of O-1(2) by its luminescence is a powerful tool that helps to understand the generation of O-1(2) during UVA exposure in solution, in vitro and in vivo. This article describes the endogenous photosensitizers, their ability to generate O-1(2) under UVA irradiation, and the detection technology to visualize the action of O-1(2).