Lung cancer incidence is now decreasing in US men. Although rates continue to increase in women, the rate of increase is declining. Most lung cancer in men and women is attributable to cigarette smoking. Histologic patterns are consistent with smoking trends for gender, race, and age. Trends in adenocarcinoma may be related to an increase in exposure to tobacco-specific nitrosamines from low-tar cigarettes. Other risk factors, including exposure to residential radon, occupational exposures, diet, and family history, have been shown to increase risk of lung cancer independent of cigarette smoking. Recent research in molecular epidemiology has greatly increased our understanding of the mechanism of lung carcinogenesis and the interactions between exposure to lung carcinogens (smoking, occupational exposures, radon), diet, and heritable variations in susceptibility.