Results of radiation therapy for malignant tumors have steadily improved, and both radiation biology and radiation physics have contributed to this improvement. As examples of such contribution, radiobiologically-elaborated continuous hyperfractionated accelerated radiotherapy (CHART) has been proven to be superior to conventional radiotherapy against non-small cell lung cancer, and a hypoxic cell sensitizer nimorazole has been proven to be effective against pharyngeal and supraglottic laryngeal cancers. Based on laboratory studies, a combination of chemotherapy and radiotherapy has been shown to be superior to radiotherapy alone in many cancers. Radiation biology has also provided important fundamental data for clinical applications of heavy ion and proton beam therapy. In the future, useful predictive assays and radioprotectors are also expected to be developed. Radiation biology should continue to contribute to the further development of clinical radiation therapy.