The two most commonly involved pathogenic bacteria in the periodontal disease are Aggregatibacter actinomycetemcomitans(A.a) and Porphyromonas gingivalis(P.g). A.a is a member of green cluster while P.g is a member of red cluster causing destruction of periodontal tissues. Their ability to invade the tissues through various virulence factors have made them gain importance in the field of periodontology. This book mentions about the history, taxonomy,prevalence,structure biochemical properties, Physical properties,site specificity and various virulence factors of these bacteria.