Diabetes mellitus is a common and growing global health problem leading to several complications. Among these periodontal diseases are considered as the sixth complication of diabetes mellitus. This article reviews the relationship between diabetes and oral health, particularly focusing on periodontal diseases, dental caries and xerostomia. There is a bidirectional interrelationship between diabetes and periodontal diseases. Periodontitis is more prevalent and severe in patients with diabetes than in normal population. Therapy of periodontal infection contributes to a positive glycaemic control management and enables reduction of the burden of complications of diabetes mellitus. Diabetics have an increased predisposition to the manifestation of oral diseases like candidiasis which is associated with poor glycaemic control and therapeutic dentures. This predisposition also contributes to xerostomia, which may be due to increased glucose levels in oral fluids or immune dysregulation.