This document summarizes the biological effects of radiation. It discusses how radiation interacts with tissues and DNA, causing damage at the cellular and genetic levels. Both deterministic and stochastic effects are explained. Acute radiation syndrome occurring in stages is described. Principles of radiation protection aim to prevent deterministic effects and limit stochastic risks. Radiation risk depends on factors like dose, exposure time and tissue sensitivity. High radiation exposure can cause acute effects while long-term low-dose exposure risks late effects like cancer.