The technologies used in nuclear medicine for diagnostic imaging have evolved over the last century, starting with Röntgen’s discovery of X rays and Becquerel’s discovery of natural radioactivity. Each decade has brough innovation in the form of new equipment, techniques, radiopharmaceuticals, advances in radionuclide production and, ultimately, better patient care. All such technologies have been developed and can only be practised safely with a clear understanding of the behaviour and principles of radiation sources and radiation detection. These central concepts of basic radiation physics and nuclear physics are described in this chapter and should provide the requisite knowledge for a more in depth understanding of the modern nuclear medicine technology discussed in subsequent chapters