Diabetes mellitus (DM) is one of the most frequent pathologies that dentists encounter, due to its high prevalence worldwide. It is diagnosed by the repeated obtaining of fasting plasma glucose levels of 126 mg/ dl or higher, or glycosylated hemoglobin of 6’5% or higher. Diabetes (especially if it is not well controlled) brings with it a greater risk of periodontal disease, which is the most frequent complication. On the other hand, the possible influence of periodontal disease on glycemic control is still not well established. Other reported manifestations are xerostomia, sialadenosis and burning mouth syndrome. With regard to dental caries, oral lichen planus and candidosis, recent studies have not revealed a significantly higher incidence in these patients. For dental treatment, the type of diabetes suffered, the treatment given for the disease, and the glycemic control status (using the glycosylated hemoglobin test) should be known. Patients should receive short morning appointments to reduce stress. The dentist has to be aware of the possible occurrence of an acute complication (hypoglycemia or hyperglycemia). Furthermore, these patients suffer from delayed wound healing and major susceptibility to infections.