Photodynamic therapy (PDT) can be defined as eradication of target pathogenic micro-organisms by reactive oxygen species produced by means of a photosensitizing compound and light of an appropriate wavelength. Thus, providing an alternative to antimicrobials in targetting periodontal pathogens and overcoming the problems associated with antimicrobials resistance. This book gives a detailed insight into Photodynamic therapy, a brief history, its mechanism of action, an effect on bacteria, fungi, and virus and also in the treatment of Peri-implantitis. The chapters have been profusely illustrated with flowcharts and clinical photographs for easy understanding of the concepts with the main objective of its application in contemporary clinical practice.