The Course provides the fundamentals of light and living organism interactions, focusing on the use of light for diagnosis and therapy. Photoproduction of energy (photosynthesis and its applications in green energy production), photophysical phenomena, light propagation in human tissues and use of light in the treatment of tumours and other pathologies are described in details. Biomedical applications related to photodiagnosis, photosensitivity, phototherapy, photodynamic therapy and photosensitizing drugs are examined. In addition, part of the course covers topics related to Optogenetics, a novel technique that uses light to control neurons, which have been genetically modified to express light-sensitive ion channels. Optogenetics uses a combination of optical and genetic techniques to control the activities of individual neurons in living tissues.