Diabetes mellitus is an important disease of the endocrine system. Many studies have associated this disease to the pathogenesis and the severity of periodontal disease. The aim of this article is to illustrate the relation between diabetes mellitus and periodontal disease. Many studies show an important association between diabetes and the pathogenesis of periodontal disease. Vascular changes caused by hyperglycemia are associated to the development of periodontal pathogens species. Moreover diabetics show an exacerbate host response with hyperproduction of inflammatory mediators and polymorphonuclear dysfunction. Diabetics with good metabolic control and patients with good oral hygiene show a reduced risk of periodontitis. In conclusion, diabetes mellitus (IDDM and NIDDM) is an important risk factor for periodontitis. Odds Ratio is 3. Diabetes mellitus determines changes in bacterial population and production of inflammatory mediators, and reduces the efficacy of the host response. Good controlled diabetes do not cause a major risk of periodontitis and improve the results of the periodontal therapy. Moreover periodontal therapy may reduce the request of insulin in diabetics. It is reasonable a two-ways relation between diabetes and periodontal disease.