External beam radiotherapy plays a significant role in the treatment of small cell lung cancer. In patients with limited stage disease (LD), thoracic irradiation improves locoregional tumor control, increases the percentage of patients achieving a complete response (CR), and produces significant increases in survival. As a result, combined modality therapy (chemotherapy/radiotherapy) is considered the treatment of choice in patients with LD. Radiotherapy plays a less significant role in extensive diseased patients, but nonetheless increases CR in the chest and has palliative benefit. Elective cranial irradiation decreases the development of central nervous system failure, thereby improving the quality of life of many patients. Its use, however, should be restricted to patients achieving CR to therapy and doses should be modest. Both whole-body and hemi-body irradiation have significant activity against the disease but should be considered strictly investigational.