Modern physics, radiation, atomic and nuclear physics have revolutionized medical diagnosis and the treatment of cancer. The work of the scientists whose discoveries fuelled this revolution is an important part of our scientific and cultural heritage. Using basic physics and simple mathematics this book shows how the discoveries of fundamental physics lead to an understanding of the important design principles of diagnosis and radiation therapy. With its carefully chosen and realistic exercises and worked examples, it provides a brief introduction and broad foundation for students and practitioners in the life sciences. This book could be used as a text for an introductory course in medical physics or biophysics. For those who are starting their careers in medical sciences or are already practitioners, it offers some interesting and useful background and an aide-memoire of the basics. For members of the public it could provide a deeper understanding of the science that informs the medical procedures that too many will be subject to, at a deeper level than the often excellent but, of necessity very basic and purely practical information available from hospitals and Web sites. The former audience may be interested in the mathematical demonstrations; the latter certainly will not be. However, for both audiences, the details of the calculations are less important than the knowledge that they can be done.