Photodynamic therapy is the subject of this interview with Dr. Steffen Rieger M.Sc. The photodynamic therapy is a light induced, athermal procedure for the inactivation of pathogenic microorganisms. Dr. Rieger faces interesting questions around this method. In the mode of action dye molecules adhere to the bacterial membrane. In a second step this photosensitizer is activated by a laser light with the fitting wave length. The dye reacts with oxygen and leads to the formation of high reactive singlet oxygen that destroys the bacterial membranes. The photodynamic therapy can be used after non-surgical and supportive periodontal therapies. It cannot be used as a replacement of antibiotics according to a Full Mouth Disinfection. But the painless procedure without risks and side effects is getting more and more accepted by the patients. For an introduction and clinical cases using different clinical procedures with the photodynamic therapy look for Dr. Rieger's lectures and case presentation, too.