The past decades have significantly widened the perspectives of the chronic oral infectious disease known as periodontitis. The disease is regarded as a bacterial infection resulting in low-grade inflammation of the periodontal tissues, and both the associated release of pro-inflammatory mediators and the presence of bacteria in the periodontal pockets, which, as the result of daily procedures, may spread after penetration of the vasculature, are possible mediators of systemic consequences. This chapter deals with the possible association between periodontitis and diabetes mellitus which is believed to possess in a two-way interrelationship.