This chapter describes semiconductor sensors used for direct X-ray detection. It also describes typical X-ray detection applications and their resulting sensor requirements. The chapter explains the traditional scintillator detectors with modern direct-conversion detector technology using semiconductor detectors. It analyzes direct-conversion sensors and contrasted silicon, germanium, gallium arsenide , cadmium telluride, and cadmium zinc telluride. The chapter discusses the requirements of the sensor electronics required to read signals generated by direct-conversion sensors. It demonstrates some examples of the usage of semiconductor sensors in NDT, medical imaging (including CT), and baggage scanning. The chapter concludes by highlighting future opportunities in product development that are offered by semiconductor sensor technology. Achieving the necessary material discrimination capabilities relies on the development of cost-effective multipixel, energy-sensitive, semiconductor sensor–based systems. There are two ways to detect X-ray radiation today: indirectly using scintillator materials and directly using semiconductor sensors.