Janus, the ancient Roman god depicted with two faces is an appropriate metaphor for light therapy. Depending on whether or not light is combined with a non-toxic photosensitizing dye (photodynamic therapy), and on the overall dose of light (energy density and power density), light is able to kill just about anything that is living such as cancers, microorganisms, blood vessels, parasites, pests, and unwanted tissues. On the opposite side of the coin (or perhaps on the other side of Janus’ face), light of the correct wavelength and at the right dose (photobiomodulation) can have exactly the opposite effect, being able to heal, regenerate, protect, revitalize, and restore any kind of dead, damaged, stressed, dying, degenerating cells, tissue, or organ system. Between the bad destroying face of photodynamic therapy, and the good healing face of photobiomodulation, it may well be concluded that “all diseases can be treated with light”. This book discusses both sides of Janus’ face in regards to light therapy.