This issue, edited by Dr. Frank Scannapieco, is devoted to "Unanswered Questions in Periodontology." Review articles will help answer clinical questions such as: Can lost bone in furcations be regenerated?; What biomarkers exist to detect active periodontal disease?; Which mouthrinse products are beneficial for plaque control and gingival health?; Do mobility and occlusal trauma impact periodontal longevity?; Can peri-implantitis be treated?; Is there a genetic basis for periodontitis?; What is the minimal sample size for studies of periodontal treatment?; Does treatment of periodontal disease influence systemic disease?; Does periodontal disease cause cancer?; Is the use of photodynamic therapy beneficial to conventional mechanical therapy in the treatment of periodontitis and peri-implantitis?;  Are radiographs the best modality to diagnose and monitor periodontitis?; Does gingival recession require surgical treatment?; and Should antibiotics be prescribed to treat chronic periodontitis?