1. The interpretation of epidemiological data of periodontal disease is difficult, due to inconsistencies in the methodology used. It is not possible, therefore, to accurately assess if the prevalence of the periodontal diseases shows a world-wide decline. As long as the disease is assessed through accumulated clinical attachment loss, retention of the natural dentition in older ages entails increased prevalence in these cohorts. Contemporary epidemiological studies should ideally employ full-mouth examination of the periodontal tissues. Partial recording estimates are generally biased, especially when the prevalence of the disease is low. 2. Early-onset periodontitis is infrequent in all populations. Adult periodontitis is rather prevalent; however, advanced disease affects limited subfractions of the population (probably less than 10 to 15%). Although prevalence figures vary with race and geographic region, in most cases, the progression pattern of the disease seems compatible with the retention of a functional dentition throughout life. 3. Of a plethora of behavioral and environmental risk markers identified by multi-variate analysis, smoking and presence of certain subgingival microorganisms have been proven to be true risk factors. The same holds true for diabetes mellitus, a systemic condition that confers a risk for periodontal disease which is independent of the effect of other significant factors. 4. In certain cases, periodontal infections appear to have a systemic impact on the host. Most recent data indicate that periodontal disease may confer risk for coronary heart disease and pre-term low birth weight.