An overview of the history of cancer and cancer treatment along with a brief introduction to the fundamental principles of Radiobiology constitute Part I of this book, which serves as the motivation for the principles of Radiation Therapy, or cancer treatment with radiation.  Part II contains the fundamental ideas from Modern Physics that form the foundation for an understanding of the approaches to treatment used in Radiation Therapy.  Finally, Part III shows the applications of Parts I and II to Medical Imaging and Radiation Therapy. This unusual introduction to Medical Physics is aimed at undergraduate physics majors along with other science majors who have taken at least one year of Physics and one year of calculus, although Medical Physics graduate students and radiation oncology residents may find this different approach to the subject illuminating. This text assumes that the instructor is a physicist who does not necessarily have a background in Medical Physics.