Energy transfer to ground state triplet molecular oxygen results in the generation of singlet molecular oxygen (1O2), which has potent oxidizing ability. Irradiation of light, notably ultraviolet A, to a photosensitizing molecule results in the generation of1O2, which is thought to play a role in causing skin damage and aging. It should also be noted that1O2is a dominant tumoricidal component that is generated during the photodynamic therapy (PDT). While type II photodynamic action generates not only1O2but also other reactive species, endoperoxides release pure1O2upon mild exposure to heat and, hence, are considered to be beneficial compounds for research purposes. Concerning target molecules,1O2preferentially reacts with unsaturated fatty acids to produce lipid peroxidation. Enzymes that contain a reactive cysteine group at the catalytic center are vulnerable to1O2exposure. Guanine base in nucleic acids is also susceptible to oxidative modification, and cells carrying DNA with oxidized guanine units may experience mutations. Since1O2is produced in various physiological reactions in addition to photodynamic reactions, overcoming technical challenges related to its detection and methods used for its generation would allow its potential functions in biological systems to be better understood.