This Part introduces the fundamental concepts underlying radiotherapy physics. It moves from the structure of matter and radioactivity (Chapter 1 and Chapter 2) to an explanation of the various interactions between radiation and matter (Chapter 3 and Chapter 4). A description of the Monte Carlo method is given in Chapter 5, as it illustrates the way in which knowledge of the detailed interactions can lead to an understanding of the transport of radiation in matter. Finally, in Chapter 6, the main dosimetric quantities are defined and their relationship is discussed in order to provide a more thorough understanding of dose-measurement methods presented in Part D and dose-calculation methods presented in Part E and Part F. Numerical values of many useful quantities are given in Part M. For complementary data on fundamental radiation physics used for medical applications the reader can refer to other more detailed textbooks (Goodwin and Rao 1977; Halliday and Resnic 1988; Bushberg et al. 1994; Cherry, Sorenson and Phelps 2003)