This paper reviews current (Fall, 1990) information related to the diagnosis of periodontal diseases. As background, principles of diagnostic decision-making and conceptual shifts during the 1970's and 1980's are reviewed in brief. "Diseases" that appeared in many classification schemes for periodontal diseases in the early 1970's--for example, "periodontosis" and "occlusal trauma"--do not appear in most current classifications. A recent (1989a) classification recommended by the American Academy of Periodontology holds that "periodontitis" includes several different diseases. There is, indeed, evidence for several different forms of periodontitis, but the AAP's classification does not conform to the principles of diagnostic decision-making because of the significant overlap between and heterogeneities within its suggested "diseases". An alternative classification is suggested, based on a concept that the periodontal diseases are mixed infections whose outcome is modified by relative effectiveness of host response. This view suggests that the most usual forms, gingivitis and adult periodontitis, normally occur in persons with essentially normal defense systems. Variation in extent or severity of disease can be understood as a function of the local infection in hosts with various degrees of compromised resistance to the infection. Early-onset periodontitis (EOP) cases could be accounted for by those where host response is abnormal to some significant degree. The greater the abnormality, the greater the extent and severity of disease might be. Localized EOP cases would be those where a relatively effective specific response intervenes to ameliorate progress of disease after the initially rapid progression. Other issues are detection of disease activity and assessment of risk for disease progression. Non-cultural bacteriological tests are available, but have not yet been shown to detect or predict activity or risk. One difficulty in reaching such proof for those or other tests has been the lack of an appropriate "gold standard" for disease activity or progression. This is being remedied by development of improved automated probes and imaging technologies. Considerable effort is being devoted to determining whether factors in gingival crevicular fluid may have diagnostic utility. More evidence is needed before clinical utility is known, but several enzymes and cytokines have potential for aiding diagnostic decisions.