Two standardized methods for measuring K-Ic in ceramics are compared for use at high temperatures (precracked beam and surface crack in flexure). Results from a third technique involving the measurement of cracks around Vickers indentations are also presented. Three dental glass-ceramics, differing primarily in microstructure, were used as model materials in this study, They emphasized the importance of microstructure in determining high-temperature K-Ic values and the challenges in measuring them. The measured fracture toughnesses decreased with temperature and increased with imbedded grain size for all three methods.