Covers essential physics concepts for understanding and calculating problems in the physical energy deposited in matter from ionizing radiation. The focus of application is on medical uses, both imaging and radiation therapy. There is more weight on radiation therapy throughout the book because of the much greater need for accuracy and precision in that field. Modern protocols can guide a person toward uncertainties of only 0.5% in the energy deposited per unit mass, the dose. In order to achieve this remarkably low level of uncertainty, one must fully understand all of the subtleties along the way. That is the purpose of this book. The aim of this work is to provide a deep understanding of all the pieces that underlie these practical protocols, and not to teach one how to use these protocols. It should be of interest to those who already have a basic education in this field, but who wish to expand and deepen their knowledge without all of the overhead of a large textbook. The information contained herein provides a more intuitive grasp of the physics concepts used in radiation dosimetry in medical applications, especially, but not limited to, radiation therapy physics.