The photodynamic therapy is a light induced, athermal procedure for the inactivation of pathogenic microorganisms. Dr. Steffen Rieger M.Sc., Germany, introduces the procedure and its application. In Part 1 he gives an overview over this painless method. The active principle of the photodynamic therapy works like that: The dye molecules adhere to the bacterial membrane. In a second step this photosensitizer is activated by a laser light with the appropriate wave length. The dye reacts with oxygen and leads to the formation of high reactive singlet oxygen that destroys the bacterial membranes. The effect is temporarily and locally limited by short life-time and operating range. The photodynamic therapy can be used after non-surgical and supportive periodontal therapies. It is not to be recommended as a replacement of antibiotics according to a Full Mouth Disinfection. The painless procedure is without risks and side effects and therefore is getting more and more popular among patients. In part 2 Dr. Rieger focuses on the integration of photodynamic therapy in the treatment concept of chronic periodontitis. For a clinical case have a look at Dr. Rieger’s Case Study about photodynamic therapy on Dental Online College.