Progress in genetic engineering has made it possible to elucidate the molecular biological abnormalities in lung cancer. Mutations in KRAS and P53 genes, loss of specific alleles, and DNA methylation of the tumor suppressor genes were the major abnormalities investigated between 1980 and the 2000s. In 2004, mutations in the epidermal growth factor receptor (EGFR) gene that cause oncogene addiction were discovered in non-small-cell lung cancers (NSCLCs), especially in adenocarcinomas. Because they are strongly associated with sensitivity to EGFR-tyrosine kinase inhibitors (EGFR-TKIs), a great deal of knowledge has been acquired in regard to both EGFR and other genes in the EGFR family and their downstream genes. Moreover, in 2007 the existence of the echinoderm microtubule-associated protein-like 4 (EML4)-anaplastic lymphoma kinase (ALK) fusion gene was discovered in NSCLC; and the same as EGFR-TKIs, ALK inhibitors are being found to be highly effective in lung cancers that have this translocation. These discoveries graphically illustrate that molecular biological findings are directly linked to the development of clinical oncology and to improving the survival rates of lung cancer patients. Here, we review the remarkable progress in molecular biological knowledge acquired thus far in regard to lung cancer, especially NSCLC, and the future possibilities.