This chapter provides the exploration of imaging science concepts, since they are fundamental concepts for any type of medical imaging—especially those associated with ionising radiation and the use of x-rays. It examines the main pretreatment imaging technologies, with a view to understanding their basic use in providing reference information for both target volume and organs at risk delineation for planning, and for the production of reference images as the basis for on-treatment geometric verification imaging. In pretreatment imaging, good spatial resolution is important for observing boundaries between tissues and organs, for example, outlining target volumes or organs at risk. Scatter is radiation that arises from interactions of the primary radiation beam with the atoms within the object being imaged. The chapter concludes with a detailed description of the on-treatment imaging technologies used with traditional C-arm linacs, including those based on nonionising radiation technologies.