Radiotherapy is a cancer treatment method that uses ionizing radiation to target tumor cells, causing DNA damage and inducing cell death. This process occurs either directly through DNA strand breaks or indirectly through the formation of free radicals. Cells in the G2 and M phases of the cell cycle are more sensitive to radiation. The effects of radiation are classified as deterministic and stochastic. The effectiveness of radiotherapy is based on biological principles such as cell radiosensitivity, repair capabilities, redistribution in the cell cycle, reoxygenation, and repopulation. Modern treatments aim to improve tumor control and patient outcomes by combining radiotherapy with immunotherapy and molecularly targeted therapies.