The purpose of this review was to assess the scientific and clinical bases for the proposed classification of periodontitis. The clinical and histopathological signs and the etiology of periodontitis were described. Cross-sectional studies were analyzed to determine when onset of periodontitis most frequently occurs in adults. In addition, the progression rates of periodontitis have been assessed from longitudinal studies. No clinical, histopathological, or microbiological features could be identified that would characterize different disease entities of chronic periodontitis. The prevalence, extent, and severity of periodontitis were found to increase continually with higher age and there was no age when onset of disease would most likely occur. The rate of periodontitis progression varies largely between patients and there is no natural threshold for distinguishing various rates of disease progression. The incidence of periodontitis unresponsive to treatment depends on pretreatment progression rate, extent and severity of disease, tooth type, smoking, high levels of putative periodontal pathogens, a deficient immune response, and the type of therapy provided. There is no scientific basis for the classification "adult periodontitis" and "refractory adult periodontitis." Extensive clinical examinations are required for the diagnosis of "rapidly progressive adult periodontitis." It appears unrealistic that these examinations can be performed routinely in clinical practice. Therefore, the classification proposed by the Organizing Committee to define adult, rapidly progressive, and refractory periodontitis as specific disease entities was replaced with a simplified classification of periodontitis based on the scientific data available.