This book covers the host responses which play a critical role in determining the course of periodontal disease. We have knowledge of the microflora associated with periodontal diseases.This book emphasizes characteristics of th host's immune response to periodontopathic bacteria which act to limit the ability of these bacteria to colonize the host, to survive in the gingival pocket, and to invade the gingiva and adjacent tissues. Other features of the immune response to periodontopathic bacteria lead to destruction of the connective tissues, alveolar bone, and other tissue components surrounding the tooth and are presented.Various host modulating agents therefore will be discussed in detail.