Diabetes mellitus is a metabolic disorder characterized by disturbed glucose regulation, manifesting primarily as chronic hyperglycemia. Today about 6% of the world's population suffers from diabetes mellitus. This metabolic disorder is known to be associated with a large number of concomitant and secondary diseases of the cardiovascular system, as well as of the joint and supporting tissue systems. It is also a recognized fact that diabetes mellitus is closely associated with diverse problems in the oral, facial and jaw regions. These facts suggest that patients with diabetes may respond to orthodontic treatment differently from those without diabetes in how the periodontium and surrounding visceral cranium react to therapy. Beginning with a clinical case report we proceed to present a general overview of the clinical features and pathophysiology of diabetes mellitus, followed by a critical discussion of the results of a systematic literature search for aspects of the disease relevant to orthodontics.