This chapter introduces basic aspects of x-ray imaging for diagnostic radiology, in the so-called diagnostic energy range (10–150 keV). These include the description of photon interaction processes in this energy range (photoelectric effect, Compton scattering, Rayleigh scattering); introductory physical aspects of x-ray image formation mechanisms (image contrast based on x-ray attenuation or phase change), for applications in general radiography and in mammography; the description of the radiation from the most common type of source for x-ray imaging, that is the x-ray tube, and description of x-ray tube spectra and their shaping through filtration. Examples of x-ray imaging with test objects are provided to illustrate the basic features of attenuation and refraction of x-rays in matter. References are made to commonly used software codes for the generation of tabulated attenuation coefficients data and for calculation of x-ray tube spectra.