This chapter discusses the variation of elastic properties and deformation mechanisms with temperature for the transition metal carbides, nitrides and diborides. These materials are fairly isotropic in their elastic response and the elastic properties decrease by about 1% per 100 K. The room temperature hardness is dominated by the resistance of the lattice to dislocation motion. Due to the limited activation energy associated with the lattice resistance, the hardness of UHTCs drops quickly with temperature with diborides showing an intermediate region of limited hardness loss whereas carbides continue to decrease in hardness at high rate. Hardness values can be converted in shear stresses needed to drive different deformation mechanisms and the latter are discussed with a view of constructing deformation mechanism maps. These help to explain that the continued decrease in hardness of the carbides is due to the poor creep resistance of transition metal carbides relative to the transition metal diborides. Another apparent difference is that creep by diffusion plays a more important role in diborides than in carbides, which has consequences for microstructural design. Finally the change in fracture resistance with temperature is discussed. Viscous materials on the grain boundaries can lead to premature loss of strength as well as a reduction in strength due to creep before fracture.