Ultra-high-temperature ceramics (UHTCs), including transition-metal carbides, nitrides, and diborides, have emerged as a class of promising structural materials for applications in extreme aerospace and energy environments. Their strong covalent-metallic bonding endows them with exceptionally high melting points, elastic moduli, and thermal stability. Nevertheless, intrinsic brittleness, limited oxidation resistance, and poor sinterability remain key challenges for the engineering application of conventional UHTCs. Recently, novel material design strategies such as multiphase composites, microstructural engineering, and compositional complexity have emerged. Among these, high-entropy UHTCs (HE-UHTCs) have attracted significant attention due to their configurational entropy, lattice distortion, and sluggish diffusion effects, which collectively enhance oxidation resistance, thermal stability, sinterability, and mechanical performance. This review summarizes the crystal chemistry, mechanical behavior, oxidation, and ablation properties of conventional UHTCs and HE-UHTCs. The four core effects of HE-UHTCs-configurational entropy, lattice distortion, sluggish diffusion, and cocktail effects-are discussed in relation to their mechanical properties and oxidation resistance. The roles of computational materials science, including density functional theory (DFT), molecular dynamics (MD), and machine learning, in composition screening and property prediction are critically reviewed. Finally, key challenges and future directions for the rational design and engineering application of UHTCs are discussed. 1 6 2026 16 11 693 693 12 6 2026 © 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license . Abstract Ultra-high-temperature ceramics (UHTCs), including transition-metal carbides, nitrides, and diborides, have emerged as a class of promising structural materials for applications in extreme aerospace and energy environments. Their strong covalent–metallic bonding endows them with exceptionally high melting points, elastic moduli, and thermal stability. UHTCs have emerged as a class of extreme-environment structural materials addressing these challenges. From the perspective of intrinsic material properties, the advantages of UHTCs arise not only from their high melting points but also from their transition-metal–nonmetal bonding characteristics. Typical UHTCs generally have melting points above 3000 °C, and some systems, such as Hf(C,N), can reach approximately 4200 °C. They also exhibit high thermal conductivity (more than 140 W·m −1 ·K −1 ) and high elastic modulus (up to 600 GPa) [ 1 ]. This mixed covalent–metallic bonding enables UHTCs to maintain structural stability under extreme thermal, mechanical, and chemical environments [ 2 ]. Crystal Structures of UHTCs UHTCs mainly include transition-metal carbides, nitrides and diborides. Although UHTCs exhibit ultra-high melting points and excellent thermal stability at the macroscopic scale, fundamental differences exist in their crystal topology, bonding directionality, and defect tolerance. These structural characteristics ultimately determine their mechanical response, thermal transport behavior, and failure mechanisms under extreme service Additionally, strong metallic interactions between boron layers facilitate electron transport within the basal plane, leading to relatively high electrical and thermal conductivity in this direction. This structure-dominated anisotropic transport gives diborides excellent high-temperature mechanical properties and thermal conduction along specific crystallographic directions, but also imposes stricter requirements on microstructural design and orientation-dependent service conditions. 2.2. Bonding–Property Relationships of UHTCs The exceptional melting points and thermal stability of UHTCs originate from the strong interactions between transition-metal d orbitals and nonmetal p orbitals. From the atomic scale, the structural stability and multiphysical properties of UHTCs come from three basic interactions: metal–nonmetal (M–X), metal–metal (M–M), and nonmetal–nonmetal (X–X) bonding. Among these interactions, M–X bonding exhibits both ionic and covalent character and plays a dominant role in determining melting temperature and high-temperature strength. M–M bonding is primarily metallic and contributes significantly to electronic and thermal conductivity. The X–X bond (especially in boride) has a decisive effect on the structural stiffness and anisotropic response. The differences in bonding configurations of different UHTC systems directly lead to systematic differences in melting point, thermal conductivity and mechanical properties. Diborides often exhibit higher thermal conductivity than carbides and nitrides due to stronger metallic bonding interactions. In contrast, carbides typically display superior mechanical stability at ultra-high temperatures because of the strong M–C covalent bonds. The high bonding energy significantly suppresses atomic diffusion and lattice vibrations, enabling these materials to retain high elastic modulus and strength even at elevated temperatures. As the transition-metal constituents or compositional complexity vary, the covalent character of M–X bonds, the degree of electron localization, and the directionality of bonding can be systematically modified, leading to significant variations in melting point, elastic modulus, and thermal transport behavior. In general, increased electron localization enhances M–X covalency, which tends to increase melting point and elastic modulus while reducing thermal conductivity. From the perspective of electron-deficient compounds, the high melting points and moduli of UHTCs are therefore governed not only by bond strength but also by valence-electron filling and electronic-structure stability.