Diabetes mellitus is an etiologically and clinically heterogeneous group of metabolic disorders that share the commonality of hyperglycemia. Long-term hyperglycemia produces tissue damage, which ultimately manifests as microvascular and macrovascular disease, and neuropathy. The presence of macrovascular disease should alert clinicians to the possibility that the patient may have coronary artery disease, particularly because coronary artery disease and myocardial ischemia are likely to be silent. Elderly patients with diabetes mellitus are also more likely to develop congestive heart failure. Patients with unstable coronary syndromes, decompensated heart failure, and symptomatic cardiac arrhythmias are at increased risk of perioperative cardiovascular complications (myocardial infarction, heart failure, and sudden death) while undergoing noncardiac procedures. In addition, clinicians must avoid the risk of hypoglycemic episodes. Oral health care providers can expect to be called upon to care for patients with this progressively debilitating disease. To provide competent care to patients with diabetes mellitus, dental clinicians must understand the disease, its treatment, and the impact the disease and its treatment may have on the patient's ability to undergo and respond to dental care.