This course will focus on the physics involved in external beam radiotherapy. We will discuss the generation of x-rays in linear accelerators and the calculation of dose. The parameters required for point dose calculations and their dependencies will be discussed. The components of a linear accelerator and their effect on dose output, scatter dose and dose distributions will be analyzed. We will also review many of the practical devices used to alter the dose distribution. Discussion on how dose transmission, scatter and absorption are dependent on the material is reviewed. The more commonly used calculation algorithms are reviewed. The course will then repeat much of the same discussion for superficial x-ray beams and Cobalt teletherapy. Principles of electron interaction in matter and dose deposition will be discussed. Properties of electron distributions, parameterization of electron beams, shielding, scattering, monitor units calculations and special electron therapy procedures will be included. Brief discussion on other particles used in radiation therapy will follow. The course will continue with radiation detection and measurement as related to radiation therapy. This will include review of many common radiation detection methods in radiation therapy as well as Bragg-Gray cavity theory for dose measurements. This is where books by Knoll and by Attix will be useful.