In the eighties of the former century it was observed that individuals had different modes of development of periodontitis, largely for unknown reasons. It was assumed that periodontal disease developed as a result of the presence of virulent micro-organisms in combination with an inadequate host response. The concept of this (im)balance in host parasite interactions developed further, in part because of new research techniques. With respect to oral bacteria it was shown that periodontal pathogens like Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis and Tannerella forsythia are risk factors for periodontitis. Furthermore, genetic polymorphisms (SNP’s) have been shown to modulate the host response against the periopathogens. Medical and lifestyle factors, e.g. diabetes mellitus, smoking, psychological stress and, to a minor extent, also hormonal changes can have an impact on the progression of periodontitis. All this information contributes to determining the individual risk profile of a patient for developing periodontitis and for estimating the prognosis of periodontal treatment and maintenance care on an individual basis.