A basic knowledge of X-ray physics is key to understanding the factors that affect both image quality and patient safety in interventional radiology. Fluoroscopy and computed tomography are common modalities used in interventional radiology that utilize X-ray beams for imaging. The absorption and scattering of X-ray photons varies depending on the material the beam interacts with; it is these differences that are key to generating image definition. However, the interaction of X-ray photons with biologic tissue can generate damaging free radicals that can have both somatic and genetic effects on the irradiated individual. This chapter provides an overview of the basic physics involved in X-ray imaging and the biological effects of ionizing radiation.