Diabetes mellitus is one of the most prevalent systemic diseases in world population and is considered a public health problem. This disorder is classified as one of the most important risk factors associated with periodontal disease. We thus undertook a literature review on the relation between diabetes and periodontal disease. Among the main results, we found that the prevalence of periodontitis is higher in diabetics than in non-diabetics, and also that the prevalence of diabetes is higher in people with periodontitis when compared to a control group. In conclusion, we established a relationship from three different hypothesis: diabetes as a risk factor for the onset and progression of periodontitis, periodontal infection as related to an aggravation of low glycemic control, and the existence of a bidirectional relationship between these two diseases. We propose a new clinical model that explains this bidirectional relationship.