This course will review the various types of electromagnetic radiation, focusing primarily on the different types of ionizing radiation. The concepts of energy, frequency and wavelength will be reviewed, as well as a quick review of ionization vs. excitation. The radiobiological principles of Linear Energy Transfer (LET) and Relative Biological Effectiveness (RBE) will be defined and explained. This presentation will also review the various effects of ionizing radiation within the body, including direct and indirect ionization. Gaining a greater understanding of these principles will allow you to better understand treatment planning rationale and the underlying premise behind beam selection.