Primary lung cancer is now the most frequent cancer in the world. Tobacco smoking still represents its predominant cause: a recent estimate attributes 80-85% of lung cancers to smoking and the epidemic of lung cancer mortality is still ongoing, with a major contribution from East European and developing countries. The study of smoking in association with lung cancer has contributed to the understanding of the carcinogenic process in humans, especially since molecular epidemiology techniques have been developed. However, the most probable carcinogenic model, still needs to be more clearly established. Presently, it can be summarised that smoking cessation is beneficial at any age, and more so when early. The contribution of passive smoking to lung cancer risk has been widely documented; its effect in terms of attributable number of cases, however, is not easily estimated at a population level. Some authors suggest that non-smoking-related lung cancer frequency is increasing over time. Exposure to environmental carcinogens with a major emphasis on those deriving from industrial processes, among which asbestos, and on air pollution due to traffic in urban areas has been advocated. Finally, a special interest has grown in the last years on possible protective factors for lung cancer, mainly diet-related (high intake of fresh fruits and vegetables), but results from the first large randomised chemoprevention trial based on alpha-tocopherol and beta-carotene treatment, dealt with "paradoxical" results.