Radiobiology is the study of how ionizing radiation interacts with biological systems and produces cellular, tissue, and whole-body effects. This course covers the mechanisms of radiation damage, DNA damage and repair, cell survival, radiosensitivity, cell cycle, oxygen effect, LET and RBE, dose–response relationships, fractionation and the 4 Rs of radiobiology. It also explains deterministic and stochastic effects, normal-tissue and tumor responses, radiation-induced cancer, and the principles of radiation protection. The course is designed to provide the essential radiobiological knowledge required for understanding and preparing for the RSO examination.