The temperature dependencies of Young's modulus (E) and fracture toughness (K-1c) of several silicon nitride-based monolithic and composite materials, are reviewed. A transition range is observed between 1130 and 1180degreesC on the E(T) curves, which is systematically 150 to 200degreesC above the T-g of oxynitride glasses of composition close to that of the intergranular glassy pockets. It is thus supposed that this transition reflects the behaviour of the interfacial glassy films. The higher the glassy phase content, the higher is the temperature sensitivity. The presence of SiC particles greatly attenuates the sensitivity. Thus, Young's modulus decreases more slowly with temperature and fracture toughness changes little up to 1300degreesC. The K-1c (T) curves exhibit four different stages which are discussed and interpreted on the basis of a theoretical model.