This chapter deals with the association of diabetes with various oral diseases, including gingivitis, periodontitis, caries, hyposalivation, candidal infection, and cancer. The incidence of gingivitis is particularly high in children and adolescents with newly discovered type 1 diabetes, and in individuals with diabetes with glycated haemoglobin values above 10%. The role of neutrophils in the development of periodontitis in general is considered protective, and impaired neutrophil function may account for an increased susceptibility to periodontitis. Indeed, neutrophil function in individuals with diabetes and periodontitis has been studied intensively. A model-based cost-effectiveness analysis of periodontal treatment among individuals with type 2 diabetes has shown that providing non-surgical periodontal treatment for people with type 2 diabetes and periodontitis would likely have meaningful public health benefits. In the oral cavity various types of malignancy may occur, with squamous cell carcinoma accounting for the vast majority.