Cigarette smoking is the largest preventable risk factor for morbidity and mortality in developed countries. Dramatic changes in the prevalence of cigarette smoking in the second half of this century in the United States (i.e., a reduction among men and an increase among women) have reduced current smoking levels to approximately one quarter of the adult population and have reduced differences in smoking prevalence and smoking-attributable diseases between the sexes. Current smoking in the United States is positively associated with younger age, lower income, reduced educational achievement, and disadvantaged neighborhood environment. Daily smokers smoke cigarettes to maintain nicotine levels in the brain, primarily to avoid the negative effects of nicotine withdrawal, but also to modulate mood. Regular smokers exhibit higher and lower levels of stress and arousal, respectively, than nonsmokers, as well as higher impulsivity and neuroticism trait values. Nicotine dependence is the single most common psychiatric diagnosis in the United States, and substance abuse, major depression, and anxiety disorders are the most prevalent psychiatric comorbid conditions associated with nicotine dependence. Studies in twins have implicated genetic factors that explain most of the variability in vulnerability to smoking and in persistence of the smoking phenotype. Future research into the causes of smoking must take into account these associated demographics, social factors, comorbid psychiatric conditions, and genetic factors to understand this complex human behavior.