Dental researchers have attempted to understand the microbial nature of oral diseases over the past 120 years. Their view of plaque and its constituent microorganisms has shifted from a specific plaque hypothesis to a non-specific plaque hypothesis and back again to a theory of specific periodontal pathogens in plaque. Changes in the way plaque and its microorganisms are viewed affect the strategies used to prevent and control periodontal diseases. In recent years, dental researchers have begun to view plaque as a biofilm. This shifting view of plaque has important implications for future efforts in research, prevention and treatment. This course addresses the various ways that dental professionals have viewed plaque throughout the years, highlighting the current view of plaque as a biofilm and the ramifications for periodontal therapy.