Gingivitis and periodontitis are the most prevalent periodontal diseases in adults. Gingivitis is characterized by inflammation of the gingiva without loss of connective tissue attachment to the teeth while periodontitis results in loss of attachment and alveolar bone and may lead to tooth loss. Gingivitis is highly prevalent in adults in the United States, and up to 70% of adults have at least mild periodontitis. In only a small proportion of adults (< 15%) does periodontitis progress to severe disease. The etiology of periodontal disease is infection with pathogenic dental plaque bacteria in a susceptible host. Strategies for preventing periodontal diseases therefore may intervene at the level of the initiation of the inflammatory process, or by preventing the progression of bone and attachment loss in periodontitis. Improved mechanical and chemical plaque control as well as improved restorative materials to facilitate plaque removal continue to enhance the patient's ability to control the plaque bacteria. Strategies to target prevention to the patients who need it most include risk factor assessment, new diagnostic methods, and further elucidation of the natural history of periodontal disease. Further study of the etiology and pathophysiology of periodontitis will aid in the prevention of further destruction through targeted use of local and systemic antibiotics and well as drugs to aid in the host response. Ultimately research may yield multivalent vaccines to be used in high-risk patients.