Recent studies regarding research on oral microorganisms and the oral diseases are presented. The normal flora of the mouth and the oral environment are first described. Dental plaque and dental caries are primary causes of oral disease. Streptococcus mutans is the major contributor in the initiation and progression of dental caries. Lactobacillus, Actinomyces, and Veillonella are other genera of bacteria linked to dental caries. Periodontitis and gingivitis are periodontal diseases that are caused by oral microorganisms. New research has indicated that various antimicrobial agents and techniques to eliminate or lessen the severity of periodontal diseases. Premature delivery of low birth weight babies in pregnant women has been strongly linked to periodontal disease. Present and future microbiological tests are available to easily determine the causative organisms for most oral diseases that help in diagnosis and treatment of a particular disease.