Cigarette smoking is a risk factor for many diseases, and recent evidence indicates that smoking adversely influences periodontal health. A number of epidemiologic studies have shown strong associations between smoking and the prevalence and severity of periodontitis, as well as interproximal bone loss. Whereas the pathogenesis of periodontitis in smokers is poorly understood, there are data suggesting defects in neutrophil function, impaired serum antibody responses to periodontal pathogens, and potentially diminished gingival fibroblast function. The prevalence and severity of periodontitis in former smokers is decreased compared with current smokers, providing evidence that smoking cessation is beneficial. Smoking markedly influences response to treatment, and a subset of smokers predominates among patients with refractory periodontitis whose disease is resistant to conventional treatment. Smokers are a high-risk group for periodontitis, and smoking history is a useful clinical predictor of future disease activity. Current estimates suggest that smoking is associated with a large proportion of periodontitis cases and constitutes a major dental public health problem. A new disease category, smoking-associated periodontitis, is proposed, given the unique characteristics of smokers with periodontitis.