In 2003, after a half century of research, numerous environmental causes of lung cancer have been identified. While most cases globally and in the United States can be attributed to cigarette smoking, lung cancer also is caused by occupational exposures and general environmental exposures. This brief review covers the causation of lung cancer by environmental agents, focusing on critical issues in 2003, along with research needs for the future. Detailed reviews are available.1–2 The regulatory and legal context around environmental causes of lung cancer merits mention, as almost all environmental agents causing lung cancer have been the focus of either regulation or litigation. For some of these agents (eg, radon or second-hand smoke), there has been controversy over several decades concerning the extent of the risks posed to the public and the need to control exposure. For these agents, the epidemiologic evidence has been a principal basis for action and, frequently, the focus of intense discussion and controversy. For active smoking, there has been litigation at several levels, as follows: individual claimants with lung cancer, attributed to either active or passive smoking; states attempting to obtain compensation from the tobacco industry for expenditures for smoking-caused diseases, including lung cancer; and national governments, also seeking to reclaim health-related expenditures. Concern about diesel exhaust as a respiratory carcinogen has led to substantial epidemiologic and toxicologic research, as well as changes in diesel engine control technology.,10 In the United States, the Environmental Protection Agency is charged with assessing the risks of a set of “hazardous air pollutants,” many being respiratory carcinogens, and workplace carcinogens are regulated by the Occupational Safety and Health Administration and the Mine Safety and Health Administration for miners. Because of the sweeping societal implications of the evidence on the environmental causes of lung cancer, research on this topic is frequently the focus of intensive and even adversarial scrutiny, and complex questions are posed regarding causation in population groups and in individuals that cannot always be answered to the needed degree of certainty.