Radiation sensors detect and convert radiation of interest (e.g. charged particles, neutrino, X- and gamma–rays) into electric charge. Reading out from radiation sensors requires highly specialized electronics. In this talk, the low-noise design techniques and circuits adopted in state-of-the-art CMOS Front-End ASICs for radiation detectors are presented. Design applications include high-energy and particle physics, energy science, space, and medical imaging. Special design challenges on CMOS devices at extreme environment. e.g. cryogenic operation and space radiation, are also discussed.