Unfavorable systemic conditions condition a low resistance of the host to the virulence of the aggressive agent, causing in a more rapid evolution of the periodontal disease and being able to lead to difficulties in the therapeutic response. The longer the duration of diabetes mellitus (DM), the greater the severity of periodontal disease and loss of insertion. The objective of the present literature review was to evaluate the relationship between periodontal disease and diabetes mellitus. The mechanisms that explain the association between diabetes and periodontal disease suggest that diabetic patients present reduced polymorphonuclear leukocyte function and chemotaxis, reduced collagen synthesis by gingival fibroblasts and glycosaminoglycans, increasing the collagenase activity of the crevicular fluid, resulting in loss of periodontal fibers and loss of alveolar bone support. Epidemiological studies have found a high degree of association between DM and periodontal disease. It has also been shown that this relationship is bidirectional, with periodontitis exerting an effect on DM. Thus, the high prevalence of periodontal disease in DM indicates the need to evaluate glucose levels in patients with periodontal disease. Intervention studies have shown that treatment of periodontal disease improves glycemic control.