Lung cancer is the result of molecular changes that occur in the cell, resulting in the deregulation of pathways which control normal cellular growth, differentiation, and apoptosis. Several of these pathways contain well-characterized proto-oncogenes and tumor suppressor genes which are found to be mutated or have abnormal expression patterns in lung cancer. The molecular changes that characterize lung cancer are complex, but it is known that cigarette smoking causes most squamous cell and small-cell carcinomas. However, the association between cigarette smoke and adenocarcinoma is less clear. Environmental factors, such as air pollutants, radon, and asbestos, likely contribute to the development of lung cancer. In this review, we discuss the major molecular abnormalities in lung cancer with a review of recent studies that begin to decipher the role that different tumor suppressor genes and oncogenes play in the pathogenesis of lung cancer. Also, we highlight the research that has identified new genes which may play a role in lung cancer pathogenesis or progression.