Physics has reduced fear and increased safety for society, largely by extending the power to see. The methods used are magnetic resonance, ionising radiation and sound, with their extensions. This textbook expounds the fundamental physics of these. It follows how they are applied by modern technology to "seeing" in clinical medicine including therapy and in other spheres of human activity such as archaeology, geophysics, security and navigation. By taking a broad view over the whole field, the book encourages comparisons, underlines the importance of public education and reaches fresh conclusions of some political importance concerning safety. This textbook has developed from a course given to third year students at Oxford and is written so that it can be used coherently as a basis for shorter courses by omitting certain chapters.