This chapter provides an introduction to basic radiation interaction concepts that are important for understanding radiation detection. Some historical perspective is also provided. It covers the SI system for physical quantities and the associated units that apply to radiation. Excitation, ionization, and other forms of energy transfer that could be used for radiation measurement are also elaborated. The chapter concludes with an overview of the remainder of the book, including chapters on various detectors used in therapy (external beams and brachytherapy), imaging, nuclear medicine, and radiation protection.