There is a growing body of scientific evidence indicating that nicotine contributes to the progression of periodontal disease, and is detrimental to healing following periodontal therapy. Smokers show a higher prevalence and greater severity of periodontal disease than non-smokers. Nicotine has toxic effects on peripheral, circulation, which cause gingival vasoconstriction. As a result, a decreased number of immune cells are available in the gingival tissue, which translates into a weakened defence-reparative system. Nicotine can also depress primary and secondary immune response by reducing the chemotactic and phagocytic activities of leukocytes. Clinically, smoking has been associated with increased pocket depths, calculus deposition,alveolar bone loss, acute necrotizing ulcerative gingivitis, and osteoporosis in postmenopausal women. Despite efforts by various investigators, the precise mechanisms underlying the effects of smoking on periodontal status and wound healing remain unresolved. This paper reviews the relationship between smoking and periodontal disease. It also includes a historical overview of the research, and a review of the effects of smoking on oral bacteria, the immune system and exocrine glands. In addition, risk assessment for periodontal disease, clinical features of smokers' periodontitis, response of smokers' periodontal disease to treatment, and ideas for future research are discussed.