Currently, most radiation detectors in medical and industrial applications are based on integrating the photons emitted from the X-ray tube for each frame using scintillators and photodiodes, an indirect technique of photon detection. This technique is vulnerable to noise due to variations in the magnitude of the electric charge generated per photon. Higher-energy photons deposit more charge in the detector than lower-energy photons so that in a quantum integrating detector, the higher-energy photons receive greater weight. This effect is undesirable in many detection applications because the higher part of the energy spectrum provides lower differential attenuation between materials, and hence, these energies yield images of low contrast. Direct conversion quantum counting detectors solve the noise problem associated with photon weighting by providing better weighting of information from X-ray quanta with different energies. In the direct conversion quantum counting system, all photons detected with energies above a certain predetermined threshold are assigned the same weight. Adding the energy windowing capability to the system theoretically eliminates the noise associated with photon weighting and decreases the required dosage by up to 50% compared to integrating systems. Semiconductor materials like CdTe, CZT, and Silicon offer the best implementation possibility of direct conversion direct detectors and are subject of this book. In this introductory chapter, we discuss the principle of operation, material challenges, detector operation physics and technology, and readout integrated circuits required to detect signals processed by semiconductor sensors. We start by describing interaction models of radiation with matter and explain the basic properties of X-ray radiation sources. Several examples of X-ray detection profiles detected by direct conversion detectors are provided.