The most common dental diseases, periodontal disease and dental caries, are chronic infections caused by bacteria of normal oral flora. When these bacteria increase in number and irritation exceeds the host defence threshold, disease arises. The human oral flora comprises more than 300 different bacteria. During the last decade approximately 10 species, mainly Gram-negative anaerobes, have been noted as putative pathogens in periodontal disease. The Gram-positive and facultatively anaerobic mutans streptococci are aetiologically the most important bacteria in dental caries. Data have rapidly increased on the association of these bacteria with certain periodontal diseases or caries, on phenotypic and genotypic characteristics, pathogenic mechanisms, antibiotic susceptibility patterns and transmission among family members. Chronic dental infections have been the focus of renewed interest because of recent advances in oral microbiology as well as in medicine. We now know that in addition to oral streptococci, recently classified, fastidious periodontal anaerobes can be detected from various extra-oral infections. Oral bacteria may spread into the blood stream through ulcerated epithelium in diseased periodontal pockets and cause transient bacteraemias, which are regarded as increased risk, especially for immunocompromised patients or persons with endoprotheses. In these patients, routine antibiotic prophylaxis is recommended for invasive dental care procedures. Also the new association between dental infections and myocardial/cerebral infarction have offered new challenges for cooperation between dental and medical researchers.