There have been many technological advances in CdTe and CdZnTe materials recently, and a new generation of advanced applications using these materials is poised to enter the commercial market. The main application area in the field of X-ray detection is provided by direct conversion detectors based on CdTe and CdZnTe sensors. Direct conversion quantum counting detectors improve performance in several diverse ways. First, the electronic noise in quantum counting can be lower because of the thresholding effect. Second, quantum counting can provide better energy weighting. Rather than increasing the weights of higher-energy photons because of the integrating effect, it is possible to increase the weights of the lower-energy photons with better contrast. Third, the fill factor of these detectors can be higher than in energy-integrating detectors as anti-reflection septa are eliminated. Photon-counting detector-based X-ray computed tomography (CT) has potential to not only improve current medical images but also enable new clinical applications. High-Z materials like CdTe and CZT offer the best implementation possibility of direct conversion detectors and are subject of this book. We discuss material challenges, detector operation physics and technology, and readout integrated circuits required to detect signal processes by CdTe and CdZnTe sensors. CdTe and CdZnTe materials are extensively used in medical imaging, industrial testing, and security applications and compare favorably against standard silicon detectors. This chapter provides an overview of material properties of CdTe, CZT, and silicon together with less used material, including perovskites. We will start this chapter with a review of basic material properties followed by a discussion on radiation detection principles of operation.