The variety of silicon nitride and silicon carbide materials which have been evolved in response to growing interest in their application in gas turbines and other engineering applications are reviewed. Recent developments in the fabrication science of hot-pressed and reaction-bonded silicon nitride are discussed. Impurities present in silicon powder are described, and their relevance to silicon nitride technology assessed. New developments with pyrolytic silicon nitride are reported. Thermal conductivity and strength properties are reviewed, and fatigue data presented. After a discussion of the design problems caused by inductility and variability, and the validity of the Weibull volume concept for specifying tensile strength, a diesel engine experiment is described in which a reaction-bonded silicon nitride piston was successfully used.