Gingival health relies on a symbiotic relationship between plaque biofilm and the host immune system. Accumulation of the biofilm leads to gingivitis and incipient dysbiosis. In susceptible individuals, incipient dysbiosis triggers an inappropriate immune response, resulting in tissue destruction and periodontitis. Therefore, pathogenic biofilm, while necessary for the development of periodontitis, is insufficient to cause the disease. The histopathological changes in periodontal disease, which are described through a series of stages, constitute a continuum. Within the periodontium, the most significant immune protective mechanism is the recruitment of the neutrophils. Where the microbial challenge is contained by host the immune response, the periodontal disease remains at the established stage but stable gingivitis stage for a substantial period. Mechanical removal of the biofilm is necessary to reduce inflammation and thus promote health. Progression to the advanced lesion occurs when the microbial challenge has overwhelmed the host defence mechanisms, resulting in tissue destruction and periodontitis. Most of the tissue damage is caused by the indirect actions of the host response and only a minority attributed to the direct actions of bacteria or their by-products. Lastly, the specific histopathological features associated with acute periodontal abscesses and necrotising periodontal diseases are also discussed.