The paper reviews the present state of knowledge in photochemistry of singlet oxygen in view of its possible applications in photobiology and photomedicine. The review elucidates the mechanism and kinetics of photosensitized reactions producing singlet oxygen. Reactions of singlet oxygen involve in particular oxidation of amino acids and amino acid residues in proteins and of nucleobases in nucleic acids. Porphyrinoid sensitizers are classified according to the charge and functional groups (anionic, cationic, amphiphilic, hydrophobic). These characteristics are essential for non-covalent interactions with biopolymers or abiotic carriers, which influence photophysical properties of the sensitizer in particular microenvironment. Finally, detection, determination and spectroscopy of singlet oxygen are surveyed and prospects of its applications briefly outlined.