Dental researchers have attempted to understand the microbial nature of oral diseases over that past 130 years. The view of plaque and its constituent microorganisms have shifted from specific plaque hypothesis to a non-specific plaque hypothesis and back again to a theory of specific periodontal pathogens in plaque. Recently dental researchers have begun to view plaque as a biofilm. The nature of a biofilm helps explain why periodontal diseases have been so difficult to prevent and treat. An improved understanding of biofilm will lead to new strategies for management of these widespread diseases.