''If it were not for the great variability among individuals medicine might as well be a science and not an art.''- William Osler. Although periodontitis does not follow Mendelian inheritance patterns, evidence is mounting of important hereditary influences. A number of aspects of the inflammatory and immune responses, that are suspected to play a role in the development of periodontal disease, have a clearly defined genetic basis. Possible associations between several of these aspects of immune defence and genotype have been discussed in the context of both family and population studies.