Diabetes mellitus is a metabolic disorder characterized by relative or absolute insulin deficiency that affects 6% of the US population. There are two main types of diabetes: type 1 is characterized by autoimmune destruction of insulin-producing cells and requires insulin treatment; type 2 is more common and results from impaired insulin function and insulin resistance. Without proper glucose control, diabetes can lead to serious long-term complications affecting many organ systems like the eyes, kidneys, heart, and nerves. Dental professionals need to consider a patient's medical history and glycemic control to minimize risks during treatment and manage emergencies like hypoglycemia or hyperglycemia.