With the increasing number of diabetics in an aging population and controversial research reports on the relationship of diabetes to periodontitis, clarification of diabetes as a risk factor for periodontitis would be helpful. This review notes variations in type, metabolic control, and duration of diabetes and highlights the results of studies that have considered these variations. Diabetics who maintained reasonably good metabolic control had not lost more teeth or experienced more periodontal attachment loss than non-diabetics, although they had more periodontal pockets. Poorly-controlled diabetics with extensive calculus on their teeth had more periodontitis and tooth loss than well-controlled diabetics or non-diabetics. Long-duration diabetics were also at greater risk for periodontitis. Mechanisms by which diabetes may contribute to periodontitis include vascular changes, neutrophil dysfunction, altered collagen synthesis, and genetic predisposition. Minimizing plaque and calculus in the oral cavity through careful self-care and regular professional care is important to reduce the risk of periodontitis in diabetics.