There are many theoretical benefits of direct conversion detectors, including decrease in noise levels, which in turn allows for better image quality or decreased radiation doses and elimination of beam-hardening artifacts. Increased precision in measuring different energy levels leads to decreased noise in material-specific and weighted images. Photon-counting scanners also facilitate multi-material decomposition. In addition, multiple parameters provide more inputs than conventional dual-energy computed tomography (CT), which can be used for more precise modeling of material-specific images, especially when decomposing images for K-edge materials, potentially improving the capabilities of spectral CT to image different combinations of contrast agents. In this chapter, we are discussing design and manufacture of readout electronics that is needed to read out very small signals produced by semiconductor sensors when they detect individual X-ray photons. Semiconductor pixelated detectors, by definition, need to have a high level of segmented multi-channel readout. Several decades ago the only way to achieve this was via massive fan-out schemes to route signals to discrete low-density electronics. However, for several years now, CMOS technology can create very dense, low-power electronics with many channels which can be bonded directly or indirectly (through common carrier PCB) to the detector, allowing for much more compact solutions. There are different requirements for the CMOS technology used for the analog front-end signal processing, as opposed to that for the digital signal processing. For the analog part of the electronics, there is a requirement for a robust technology that has low electronic noise and high dynamic range, typically requiring large power supply voltages. Digital signal processing in turn requires very high speed and high density that is more compatible with the more modern low voltage supply, deep submicron processes. The purpose of this chapter is to present a review of available readout channel integrated circuit (ROIC) photon-counting ASICs.