Wide-bandgap (WBG) semiconductors, such as SiC and GaN, are prevalent in the arena of power electronics, thanks to the high breakdown field. In addition to power electronics, there has been increasing interest in advancing WBG semiconductors for electronics operating in extreme environments, such as ionizing radiation, energetic particles, and high temperatures. Hoping to further the advantages of a wider bandgap, ultrawide-bandgap (UWBG) semiconductors, such as Ga2O3, B-Al-Ga-N, and diamond, are also being investigated. This special topic collection covers broad research subtopics on radiation effects in (U)WBG semiconductors, (U)WBG semiconductors operating at extreme temperatures, as well as some recent progress in (U)WBG semiconductor physics, materials synthesis, device fabrication, and electrical/structural characterization.