This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of ππ* excited triplet states by O₂ proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O₂. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O₂ are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O₂. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.