Anti microbial photodynamic therapy is one of the most upcoming treatment approaches in field on dentistry. Applications of PDT in dentistry are growing rapidly: the treatment of oral cancer, bacterial and fungal infection therapies, and the photodynamic diagnosis (PDD) of the malignant transformation of oral lesions. Periodontitis as known is polymicrobial disease and this novel therapeutic approach sounds promising in eliminating the microorganisms when usedin adjunct to conventional debridement methods. This technique involves application of a photosensitizer and activating it with a light source of specific wavelength. This system in presence of oxygen creates reactive oxygen species that exerts the classic photodynamic reaction. The advantage of this new approach includes rapid bacterial elimination, minimal chance of resistance development and safety of adjacent host tissue and normal microflora. This review discusses about the general principles, mechanism of photodynamic therapy with additional highlights on its application in periodontal diseases.