Lung cancer is the second most common cancer in both sexes and the leading cause of cancer deaths in the United States, killing nearly three times as many men as prostate cancer and about twice as many women as breast cancer. This chapter reviews the most important aspects of epidemiology, diagnosis, treatment, and prevention of lung cancer. The term lung cancer refers to carcinoma arising from the respiratory epithelium. The four basic histologic types of lung cancer are adenocarcinoma, squamous cell carcinoma, small cell carcinoma, and large cell carcinoma. A widely used classification divides lung cancer into two major groups: small-cell lung cancer (SCLC) and non-small-cell lung cancer (NSCLC). The incidence of lung cancer correlates with the prevalence of cigarette smoking. In addition to tobacco smoking, exposure to potential carcinogens in the workplace or environment increases the risk for the development of lung cancer.