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Like surgery, radiotherapy is a local modality for cancer treatment. It exploits the inherent greater radiosensitivity of tumor compared to adjacent normal tissues. Ionizing radiation, usually x-rays or gamma-rays (most commonly cobalt-60), are used to destroy localized populations of cancer cells. Through ionization of water molecules the radiation produces free radicals and oxidants within the target cells. These chemically reactive agents break and damage DNA molecules leading to altered transcription and defective repair, which lead to cell death. Hypoxic tissues are relatively resistant to radiation damage. External beam therapy (teletherapy) is given from a source outside the body, such as a linear accelerator. Brachytherapy delivers radiation from sealed radioactive materials inserted in or near a tumor. The latter include intracavitary inserts for cervical or vaginal cancers, and interstitial implants for prostate cancer. Radionuclides like 131-iodine (for thyroid cancer) or 89-strontium (for bony metastases) are also used. About half of cancer patients receive radiotherapy in countries where good facilities are readily available. For selected tumors it can be curative as the sole treatment modality or when used in multimodality strategies with surgery. Often it is used for palliation for bone and brain metastases and alleviation of bleeding, pain and obstruction. Urgent radiotherapy is used to treat impending spinal cord compression, obstructed airway or superior vena caval obstruction. Click the hyperlink on the slide for more detail.