In this review, we present the current knowledge of the interplay between periodontal diseases and diabetes through the evaluation of randomized control and longitudinal cohort studies published in the past 15 years. Current data support the conclusion that diabetic patients are at increased risk for periodontal diseases, and that patients with poorly controlled diabetes are at risk for severe periodontitis. This results in the destruction of oral connective tissue and generalized bone loss, leading ultimately to tooth loss. Although the effect of periodontal disease on glycemic control in type 1 diabetic patients is controversial, evidence does show a direct correlation between periodontal health and glycemic control in type 2 diabetic patients. Furthermore, several studies have demonstrated the beneficial effect of periodontal treatment on metabolic control of type 2 diabetic patients.