Diabetes mellitus is a metabolic disease characterized by abnormalities in the regulation of blood glucose levels. Average blood glucose levels over a longer period of time (2-3 months) can be assessed by measuring the HbA1c levels. Glycated hemoglobin is determined with the HbA1c levels. An HbA1c of over 7% may lead to serious general health complications, particularly when it is not or cannot be well regulated. Elevated blood glucose levels may also lead to oral health complications. Epidemiological studies have shown that periodontitis, salivary disorders, dental caries, tooth loss, edentulousness, peri-implant problems and (pre)malignant disorders occur more frequently in patients with diabetes mellitus compared to non-diabetic patients. Non-surgical treatment of periodontitis may have a positive and clinically significant effect on blood glucose levels. The actual magnitude of the association between diabetes mellitus and oral health complications is still unclear due to methodological limitations of research carried out so far.