The first words of creation ?Fiat Lux?; ?let there be light? indicate the primary and ubiquitous nature of photobiology. This special issue highlights recent advances and the importance of Photodynamic therapy. At last it is proving to be a highly promising method both for the detection and treatment of cancer. All existence has been highly dependent on the light driven basic reactions exploited both in photodynamic therapy and photodiagnosis. These reactions are widespread in nature, and the clinical exploitation is proving a very attractive method of treatment of many cancers.