Recent studies of the etiology, pathogenesis, prevention, and therapy of periodontal diseases have altered our concepts of these diseases. For the last two decades, much of the work in the basic sciences related to periodontal disease has been descriptive: defining the structures of the periodontium, their changes in disease, the nature of the associated microbiota, some aspects of the host’s immunologic response to the microbial challenge, and some of the potential mediators or mechanisms of destruction. Such data clearly indicate that the major mechanisms of destruction of the periodontium are initiated by bacteria in dental plaque.