The term periodontal refers to any of the bony and soft tissues that support the teeth. Diseases of these supporting tissues affect the majority of the adult population throughout the world and are the major cause of loss of teeth in adults. The most common form of periodontal disease and the major consideration of this chapter is chronic adult periodontitis (CAP). The pathologic process(es) involved in CAP are initiated and sustained by bacteria in the dental plaque adherent to the teeth. In the absence of these bacteria, the disease can neither begin nor progress, but the destructive process of CAP does not appear to require the presence of organisms within the periodontal tissues. Thus, seman-tically, CAP is more properly classified as an infestation than an infection. As with other pathological infestations, bacterial products initiate the host response in the tissues; this response is the major source of destruction. Two of the most interesting aspects of the study of this disease concern the nature of the ecologie development of the initiating mixed bacterial flora and the mechanisms involved in the breakdown of supporting tissues by the host response. Indeed, much of the knowledge gained by researchers interested in the immunopathology of CAP is applicable to the mechanisms of tissue destruction in other chronic inflammatory diseases such as rheumatoid arthritis (Snyderman and McCarty, 1982; Decker et al., 1984).