Photodynamic therapy (PDT) combines visible light illumination and certain nontoxic dyes known as photosensitizers (PS) . When PS are irradiated by light of appropriate wavelength, the absorbed energy carried by photons leads to the activation of the PS from the ground state to the excited singlet state (Figure 16.1). is excited singlet state may then undergo intersystem crossing to the slightly lower energy but longer-lived triplet state . e latter may then react further by one or both of two pathways. Type I pathway leads to electron transfer reactions to produce radicals, while type II pathway leads to energy transfer reactions to produce excited-state, singlet molecular oxygen. Both of these processes are oxygen dependent and lead to the production of highly toxic reactive oxygen species (ROS). e ROS can readily react with biological molecules such as proteins, lipids, and nucleic acids and therefore can lead to their oxidation, subsequent damage, and activation of apoptosis .