Diamond has emerged as a benchmark ultra-wide-bandgap (UWBG) semiconductor for radiation detection and optoelectronic applications owing to its exceptional physical, electronic, and optical properties. With a wide bandgap of approximately 5.47 eV, high carrier mobility, superior thermal conductivity, excellent radiation hardness, and outstanding optical transparency, diamond surpasses conventional semiconductors such as Si and GaAs, and even other UWBG materials including SiC and Ga2O3, in several key figures of merit. Early radiation detector applications relied primarily on natural diamond; however, uncontrolled impurity concentrations and defect densities severely constrained device reproducibility and scalability. In recent years, the development of diamond synthesis techniques via chemical vapor deposition (CVD) has led to investigations into diamond electronic applications, as large-area (centimeter scale) production of single-crystal diamond can be realized, and the impurity concentration can be controlled. By 2025, advancements in CVD techniques have enabled the production of large-area single-crystal diamonds with growth rates up to 200 μm/h and enhanced quality for electronic-grade applications. Diamond radiation detectors therefore represent one of the research projects that underwent some decades of investigation and are still being under study. In this chapter, we summarize the detection mechanisms in two different types of diamond detectors: (1) diamond photoconductors and (2) Schottky barrier photodiodes, which depend strongly on the contact properties between the electrode materials and diamond surface. Meanwhile, the main features of diamond detectors, like responsivity, quantum efficiency, charge collection efficiency, charge collection distance, and others, are introduced and discussed. Recent breakthroughs include single-crystal diamond nanowires for solar-blind photodetectors with record-breaking UV sensitivity and thermal resistance. The remarkable properties of diamond promise several opportunities in a diverse range of application fields, whereas the daunting technical challenges, such as relating to size, doping, and manufacturing scalability, are still in the way of high technology commercialization, especially in electronics. Nevertheless, with new developments in materials synthesis, and research and practical applications, the outlook for diamond is exciting, including hybrid photonics for quantum technologies and advancements in n-type doping.