This chapter provides a brief overview of the principles of radiation therapy. The topics to be discussed include the physical aspects of how radiation works (ionization, radiation interactions) and how it is delivered (treatment machines, treatment planning, and brachytherapy). Recent relevant techniques of radiation oncology, such as conformal and stereotactic radiation therapy, also will be presented. These topics are not covered in great technical detail. It is hoped that a basic understanding of radiation treatment will benefit those practicing in other disciplines of cancer management. This chapter does not address the principles of radiobiology, which guide radiation oncologists in determining issues of treatment time, dose, and fractionation or in combining radiation with sensitizers, protectors, and chemotherapy or hormones.