Periodontal diseases are inflammatory and destructive diseases of the dentogingival complex associated with specific periodontal pathogens inhabiting periodontal pockets. Periodontal diseases lead to damage of the periodontal tissues supporting the teeth (bone and connective tissue) and affect the quality of life of the affected individuals: poor alimentation, tooth loss, social and financial problems. Although it is generally considered that the disease has multifactorial etiology, data show that some specific Gram-negative microorganisms in the subgingival plaque biofilm play a major role in the initiation and progression of periodontitis. Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia form a consortium in the subgingival biofilm and are regarded as the principal periodontopathogenic bacteria. Other microorganisms that have been implicated as predominant species in the disease process which are also discussed in this review.