Diabetes mellitus (DM) is a metabolic disease characterized by an increased blood glucose level, while periodontal disease is mainly characterized by the destruction of tooth support tissues. Detailed investigation is warranted to consider these highly prevalent chronic diseases together and analyze their mutual influence. Over the years, various biologically plausible mechanisms have been established for a common inflammatory etiopathogenesis of these diseases. Numerous epidemiological studies have found a high degree of association between DM and periodontal disease, and periodontal disease has even been proposed as a sixth complication of DM. It has also been demonstrated that this relationship is bidirectional, with periodontitis exerting an effect on DM. These findings have diagnostic and therapeutic implications. Thus, the high prevalence of periodontal disease in DM indicates the need to evaluate glucose levels in periodontal patients. Conversely, intervention studies have demonstrated that the treatment of periodontal disease improves the glycemic control of DM patients.