Periodontal diseases are infections. They have a number of properties in common with infections in other parts of the body, but have unique features resulting from the passage of the tooth through the soft tissue integument into the oral cavity. The tooth provides a solid, non-shedding surface for the colonization of potentially pathogenic bacterial species as well as a wide range of host-compatible species. Periodontal pathogens frequently colonize the periodontal area for prolonged periods of time prior to disease initiation. Disease is caused by a finite set of bacterial species leading to the development of multiple periodontal diseases. Disease occurs at individual periodontal sites and leaves an historical record of the damage to the periodontium in the form of periodontal attachment or bone loss. This feature provides difficulty in investigation since it is often unclear whether the infectious process is presently occurring or is a reflection of past destruction. Control of periodontal diseases is most effectively performed by eliminating or suppressing the organisms that cause them and establishing a host compatible microbiota. When tissue damage has been extensive, regenerative or replacement procedures should be instituted after the infection has been controlled.