Radiation biology is the scientific study of the series of events that occur in cells after the absorption of energy from ionizing radiation, including intracellular ionization, DNA damage, cell repair, and apoptosis. With a century of development, radiation therapy has been extensively applied in the clinical treatment of cancer. This chapter will also explore the synergistic effects of combining radiotherapy with immunotherapy, a rapidly evolving field in cancer treatment. Radiation not only directly damages tumor cells but also modulates the tumor microenvironment to enhance the killing power of immune cells. Immune radiation therapy has the potential to overcome the limitations of traditional treatments in certain types of tumors, offering significant prospects for treating diseases and improving prognoses. This article also presents an outlook on the future development of radiobiology in terms of technological innovation, the limitation of side effects, and so on.