Radiotherapy involves the use of ionising radiation to treat cancer. It is one of the primary cancer treatment modalities today. The other treatment modalities are surgery and chemotherapy. Radiation can be delivered externally or internally to the treatment site. It involves high doses and generally uses high energy photon radiation produced by a medical linear accelerator. Radioactive sources are also used for treatment. Currently, advanced radiotherapy involves intensity modulation and image guidance. Special techniques like stereotactic radiosurgery are used to treat small lesions. The use of highly conformal techniques in radiotherapy today helps to reduce healthy tissue damage while delivering the prescribed dose to the tumour.