Diabetes mellitus (DM) is diagnosed in 3% to 4% of the population. This metabolic disorder is characterized by hyperglycemia caused by the body's deficient management of insulin. Two main types of DM exist: type 1 is a total deficiency in insulin secretion, and type 2 is a combination of resistance to insulin action and inadequate compensatory insulin secretion. Diagnosis and monitoring of DM is based on blood glucose concentration or glycosylated hemoglobin concentration. The dental practitioner should be aware of the oral manifestations of DM in order to spot initial symptoms of the disease. When treating DM patients, the practitioner must understand the consequences of the disease in relation to dental treatment. In this review, we examine the implications of DM when orthodontic treatment is planned.