Tobacco smoking, an environmental factor, is now acknowledged as a major risk factor in the development and progression of variety of periodontal diseases. Smokers are almost thrice as likely to have severe periodontitis compared to nonsmokers and also associated with 90% of refractory periodontitis. Classification of smokers, effect of smoking on the prevalence and severity of periodontal disease, effect of smoking on the etiology and pathogenesis of periodontal disease, effects of tobacco smoke on dental structures, effect of smoking on the response to periodontal therapy, and effect of smoking cessation are discussed in this chapter.