Achievements in bacteriology and immunology have significantly contributed to the understanding about the etiology and pathogenesis of periodontal diseases. Studies have demonstrated that interactions between bacteria and the immune system play a central role in the etiology of periodontal disease. In the immune system four components can be distinguished according to their function in the periodontium. These include a) the secretory system; b) the polymorphonuclear leukocytes, antibody and complement system; c) lymphocytes and monocytes; d) the immunoregulatory system. Polymorphonuclear leukocytes are of utmost importance for the maintenance of periodontal health. In neutrophil defects severe periodontal diseases are frequently found. Prepubertal periodontitis, juvenile periodontitis and rapidly progressing periodontitis are examples for the periodontal manifestation of neutrophil defects. Therapy of periodontal disease that is related to immune defects requires a modified treatment plan. Efforts are being made to extend the diagnosis, prevention and therapy of periodontal diseases. Presently, biochemical, immunological and microbiological methods are being evaluated for their clinical applicability. Preliminary results indicate that, in the future, biochemical markers in the gingival crevicular fluid may be used for the diagnosis of active sites. Therapy may be enhanced by the administration of non-steroidal anti-inflammatory drugs and specific antimicrobials. However, before these methods can be applied in clinical periodontics, further long-term studies are needed.