Tobacco use, primarily associated with cigarette smoking, is the largest preventable cause of cancer mortality, responsible for approximately one-third of all cancer deaths. Approximately 85% of lung cancers result from smoking, with an additional fraction caused by secondhand smoke exposure in nonsmokers. The risk of lung cancer is dose dependent, but can be dramatically reduced with tobacco cessation, especially if the person discontinues smoking early in life. The increase in lung cancer incidence in different countries around in the world parallels changes in cigarette consumption. Lung cancer risks are not reduced by switching to filters or low-tar/low-nicotine cigarettes. In patients with cancer, continued tobacco use after diagnosis is associated with poor therapeutic outcomes including increased treatment-related toxicity, increased risk of second primary cancer, decreased quality of life, and decreased survival. Tobacco cessation in patients with cancer may improve cancer treatment outcomes, but cessation support is often not provided by oncologists. Reducing the health related effects of tobacco requires coordinated efforts to reduce exposure to tobacco, accurately assess tobacco use in clinical settings, and increase access to tobacco cessation support. Lung cancer screening and coordinated international tobacco control efforts offer the promise to dramatically reduce lung cancer mortality in the coming decades.