Radiotherapy or RT is a medical technique used for the treatment of cancer particularly, to kill or control the growth of malignant cells. The procedure uses ionizing radiation and is delivered by a linear accelerator. The radiation damages the DNA of cancerous tissue thereby leading to cellular death. Besides the tumor itself, the radiation may target the draining lymph involved with the tumor. RT is performed when cancer is localized to one area of the body. It can also be a part of adjuvant therapy for preventing tumor recurrence after surgery of a primary malignant tumor. Often, RT is used in conjunction with chemotherapy, and may also be used as palliative treatment. This book includes some of the vital pieces of work being conducted across the world, on various topics related to the theory and practice of radiotherapy. It provides significant information of this discipline to help develop a good understanding of RT and its implications, applications and consequences. It will prove to be immensely beneficial to students and researchers in the fields of oncology and radiation oncology.