Prospective studies have been conducted around the world to investigate the natural progression of periodontitis. Eight populations, with at least five years of observation, have been investigated in Norway, Sri Lanka, China, Indonesia, Australia, Germany, New Zealand, and Brazil. The findings from each population were based on the assessments of clinical periodontal parameters, microbiological samplings, socioeconomic status, tobacco use, and daily oral hygiene. Periodontitis was not commonly seen in the younger cohorts. With increasing age, however, the prevalence and severity of periodontitis rose significantly, although there was substantial variance within this general finding among individual cohorts. While the accumulation of plaque and subgingival calculus appears to be primarily accountable for the onset and progression of periodontitis, it is also necessary to consider the individual’s susceptibility and their response to the exposure of common risk factors such as smoking, as well as their socioeconomic status, in order to understand the natural progression of the disease.