This lecture is part of MedPhys 4D03 lecture series for the Fall 2020 semester. This lecture provides a brief introduction to the atomic structures. Discusses the importance of ionizing radiation and presents the two main photon interaction type: photoelectric effect and Compton effect. Production of X-rays is also explored, including the definition of the most important quantities, such as X-ray spectrum, filtering, half-value layer, beam hardening.