Diabetes and periodontal disease are two seemingly unrelated disorders whose complex interrelationship is only now being clarified. How does diabetes, a metabolic disease, affect the periodontium? Perhaps even more interesting for the diabetic patient: How does periodontal disease, a bacterial infection that results in inflammation and tissue destruction in the oral cavity, impact the metabolic control of diabetes? Evidence clearly supports diabetes as a risk factor for periodontal disease. The mechanisms by which diabetes adversely influences the periodontium are in many ways similar to the mechanisms responsible for the classic complications of diabetes. In recent years, research has demonstrated that the relationship between periodontal disease and diabetes is more complex than previously realized. Not only does diabetes affect the periodontium, but periodontal infection can adversely impact glycemic control in diabetes. Periodontal treatment that decreases the bacterial challenge and the resulting inflammatory periodontal destruction can improve glycemia in some diabetic patients. These research findings are of importance to diabetic patients and their health care providers, because the findings support the need for examination, diagnosis, treatment, and prevention of periodontal infection.