Lung cancer is the most common thoracic malignancy caused by exposures at work and in the environment. The most unique thoracic malignancy is mesothelioma, because it is relatively rare and one of only a few neoplasms for which one specific inciting agent-asbestos-has been identified. Based on epidemiologic studies, approximately 15% of lung cancers in men and 5% of lung cancers in women are caused by occupational exposures. The International Agency for Research on Cancer has devised a rating system by which, based on animal and human data, they assign an agent, mixture, or exposure circumstance to one of five categories, ranging from group 1 (agent is carcinogenic to humans) to group 4 (agent is probably not carcinogenic to humans). Group 1 pulmonary carcinogens reviewed in this article include arsenic, asbestos, beryllium, bis (chloromethyl) ether, cadmium, chromium (IV), mustard gas, nickel, radon, and silica. The clinical presentation and pathology of lung cancers and mesothelioma caused by such exposures do not differ from those of cancers caused by other factors. The key to the recognition of a thoracic malignancy caused by workplace or environmental exposures is clinical suspicion and consideration of all causes for the disease present. Recognition of an exposure-related case of lung cancer or mesothelioma can aid in the identification of excess risk for a whole workforce or community and can lead to actions to reduce exposure, thus preventing future cases. In addition, such recognition allows the individuals struck by devastating illness to exercise their legal rights to compensation if so desired.