Between 3-12 weeks after the beginning of supragingival plaque formation, a distinctive subgingival microflora predominantly made up of gram-negative, anaerobic bacteria and including some motile species, becomes established. In order to establish in a periodontal site, a species must be able to attach to one of several surfaces including the tooth (or host derived substances adhering to the tooth), the sulcular or pocket epithelium, or other bacterial species that are attached to these surfaces (Socransky and Haffajee 1991). Bacterial adhesion has demonstrated specificity in the mechanisms involved and studies have shown that there is a diversity of receptors on tooth surfaces, epithelial or other host cells and other bacteria. Recent studies have described bacterial complexes that are present in subgingival plaque and these studies are likely to help in current understanding of the complex ecology observed in dental plaque biofilm (Socransky, Haffajee et al. 1998). Bacterial interactions play important roles in species survival. Some interspecies relationships are favourable, in that one species produces growth factors for, or facilitates attachment of, another species. Other relationships are antagonistic due to competition for nutrients and binding sites, or to the production of substances that limit or prevent the growth of another species (Socransky and Haffajee 1991). A number of different bacterial interactions within plaque biofilm have been discussed. In the last 30-40 years, a vast amount of evidence has been published to suggest that bacteria are the primary aetiological agents of periodontal diseases. In the 1950s and early 1960s, periodontal treatment was based on the non-specific plaque hypothesis. However, the non-specific plaque hypothesis gave way after studies suggested that not all organisms in plaque are equally capable of causing destructive periodontal disease. Thus the concept of specificity re-emerged. Criteria for defining periodontal pathogens have been developed and include association, elimination, host response, virulence factors, animal studies and risk assessment (Haffajee and Socransky 1994). Until recently there were few consensus periodontal pathogens and trying to discriminate pathogenic from non-pathogenic species has been a difficult task for dental researchers for a variety of reasons. A discussion of the specific microbiota associated with gingivitis, chronic and aggressive periodontitis, NUG, HIV-associated periodontitis and implantitis has been presented. The bacteria associated with periodontal diseases are predominantly gram-negative anaerobic bacteria and may include A. actinomycetemcomitans, P. gingivalis, P. intermedia, B. forsythus, C. rectus, E. nodatum, P. micros, S. intermedius and Treponema sp. The bacterial numbers associated with disease are up to 10(5) times larger than those associated with health.