This book draws on experience teaching medical physics graduate students and residents to offer a self-contained and rigorous introduction to the physics of X ray tubes, medical linear accelerators (Linacs), X ray photon-counting detectors (PCDs), and X-ray energy-integrating detectors. It directly addresses the conceptual challenges learners face in understanding the operating principles of PCDs and Linacs, using an approach that is intuitive without sacrificing depth. The book is organized into six focused chapters covering basic semiconductor physics, semiconductor detector structure and elements, semiconductor photon counting detector (PCD) physics, physics of inorganic scintillators and scintillator image detectors, physics in x-ray tubes, and waveguide physics in medical Linacs. Core physical principles and governing laws are developed through first principles-based derivations, supported by schematic illustrations and real-world examples that reinforce both conceptual understanding and practical application. Written for medical physics and bioengineering students, residents, researchers, and educators, the book assumes no prior background in semiconductor physics while providing a rigorous explanation of how modern X-ray detectors, tubes, and Linac systems work. By strengthening fundamental understanding, it equips readers to master current technologies and contribute to future innovation in X ray sources and detectors.