The influence of diabetes mellitus has its effect on general health including the oral health. Among all the systemic conditions, diabetes is the strongest risk factor for periodontitis, leading to increase prevalence, severity, and progression of the disease. Periodontal disease is considered to be the sixth complication of diabetes. The diabetic state impairs the gingival fibroblast synthesis of collagen and glycosaminoglycan, enhances crevicular fluid collagenolytic activity, results in the loss of periodontal fibres, loss of the alveolar supporting bone, loosening and finally exfoliation of the teeth. Research has demonstrated the presence of elevated levels of pro-inflammatory mediators in the gingival crevicular fluid of periodontal pockets in patients with poorly controlled diabetes, compared with people without the condition or those with well controlled diabetes, resulting in significant periodontal destruction with an equivalent bacterial challenge. Oral hygiene of the patients with diabetes must be optimised to prevent further stimulation of an already activated host response. In this book we have reviewed the relationship of diabetes mellitus and periodontitis.