Diabetes mellitus is the most common disease of the endocrine system. There are two basic types of the disease: type 1 (juvenile diabetes), caused by insulin deficiency, and type 2 (adult-onset diabetes), characterized by insulin resistance. Secondary diabetes encompasses a series of conditions that cause symptoms of diabetes indirectly. Hormonal imbalances result in metabolic disorders which lead to acute complications. Pathological changes in all the blood vessels, nerves with impaired sensory and motor functions, pancreas, kidneys, eyes, mucosa, and the oral mucosa included, are inevitable. Oral manifestations of diabetes are a result of an altered metabolism and of the changes in blood vessels and nerves. Changes in the oral cavity include a typical foetor ex ore, frequent infections (Candida albicans), increased vulnerability, a tendency to periodontal diseases, stomatopyrosis and xerostomia. The symptoms in the oral cavity appear when diabetes is not under control; therefore, it is very important to bring the disease to the compensated stage before starting the dental treatment, unless it is an emergency. It is important to know how to approach a patient with diabetes and how to obtain their medical history properly in order to find out how long they have been suffering from diabetes, whether they are taking any medications and if they under medical control. Patients suffering from diabetes may get into the state of hypoglycemia or hyperglycemia and it is extremely important to be able to recognize such conditions and provide the patients with appropriate assistance.