Radiation therapy is the medical use of ionizing radiation as part of cancer treatment to control malignant cells. Total body irradiation (TBI) is a radiotherapy technique used to prepare the body to receive a bone marrow transplant. Radiotherapy is used for the treatment of malignant tumors (cancer), and may be used as the primary therapy. It is also common to combine radiotherapy with surgery, chemotherapy, hormone therapy or some mixture of the three. This chapter will go on to state the mechanism of action of radiation therapy; it then briefly describes the amount of radiation used in radiation therapy. Effect on different types of cancer are then broadly discussed then followed by types of radiation therapy.