Diabetes mellitus is a syndrome of metabolic disorders of carbohydrates, fats and proteins. It is caused by insufficient secretion of insulin or reduced tissue sensitivity to insulin. The primary characteristic of the disease are elevated blood glucose levels. Uncontrolled hyperglycemia is a cause of complications of a number of organic systems including eyes, nerves, kidneys and blood vessels. Diabetes as a systemic disease includes the changes of the oral structure as an integral part of the organism. The most common manifestations are halitosis, alveolar bone destruction, xerostomia, stomatopyrosis, inflamed oral mucosa with injuries, increased tendency to infections (candidiasis), increased risk of developing caries, gingivitis and periodontitis, and increased exudation from gingival sulcus. It is extremely important for dentists to know how to approach a patient with diabetes. It is important to take a correct anamnesis from which you can find out how long a patient is suffering from diabetes, whether he or she is taking medications or if he is under medical supervision. Such patients may fall into the state of hypo or hyperglycemia, and it is very important to recognize such conditions and provide appropriate assistance.