[en] This article is designed to refresh and remind those of you who did physics many years ago; it is not intended for those of you who are experts in the field. All of this information is readily available from a wide variety of different texts; it is not new material, but hopefully brings together important issues in one place, in a suitable and appropriate manner. This paper deals with issues that I believe are the most important physical concepts to be understood by practicing radiographers. These are the 'Interactions' and a firm understanding of these will explain: why the appropriate choice of kVp is important to achieve good image quality and low patient dose?; why a very low energy kVp is required for mammography?; why high atomic numbers are preferred for contrast studies?; why grids should be used for high kVp and not low kVp examinations?; and why lead is the best radiation shield at diagnostic energies? Copyright (2003) Australian Institute of Radiography