The temperature dependences of Young's modulus (E) and fracture toughness (K_1c), as well as the flow behavior, including stress relaxation, creep and superplasticity of several silicon nitride-based materials, monoliths and composites, are reviewed. A transition range between a low softening rate and a higher one, which coincides with the onset of creep ductility, is observed between 1080 and 1150°C on the E(T) curves and is attributed to the behavior of the secondary glassy phases. The higher the Y/Al ratio or the SiC content, the higher the transition temperature. The K_1c(T) curves exhibit four different stages which were discussed and interpreted through a theoretical analysis, warning against the frequent confusion between the intrinsic and the apparent (experimentally accessible) toughness. Reliable creep resistant- or inversely superplastic-ceramics are now available. However, the high temperature deformation mechanisms are not well understood yet. Non-Newtonian flow regimes and the pronounced tension/compression flow asymmetry are still intriguing. Today, it can be anticipated that with the development of materials containing low amounts of highly refractory grain boundary phases, ceramists are facing a situation that places them closer to metallurgists, and which should allow them to derive more benefit from the exceptional intrinsic properties of covalent crystals such as Si₃N₄. Consequently, grains should play a more and more important role on the high temperature mechanical behavior.