In this book, two non-thermal regimens of light interaction with biological tissue that are used for diagnostic and therapeutic methods are discussed. These are photodynamic therapy (PDT) for the treatment of periodontal disease and diffuse reflectance spectroscopy (DRS) for periodontal inflammation detection. The first part of the work deals with the development of a non-invasive diagnostic tool that includes point monitoring and imaging devices. Clinical trials were conducted to demonstrate the efficacy of the diagnostic device used. In the second part pre-clinical and clinical trials were conducted to demonstrate the efficacy of antimicrobial photodynamic therapy in periodontitis treatment.