This book addresses the question 'What is physics for?' Physics has provided many answers for mankind by extending his ability to see. Modern technology has enabled the power of physics to see into objects to be used in archaeology, medicine including therapy, geophysics, forensics and other spheres important to the good of society. The book looks at the fundamental physics of the various methods and how they are used by technology. These methods are magnetic resonance, ionising radiation and sound. By taking a broad view over the whole field it encourages comparisons, but also addresses questions of risk and benefit to society from a fundamental viewpoint. 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 shortened courses by omitting a number of chapters.