Strain and failure at interfaces often present a limitation to the performance of engineering systems and materials at high temperatures, including spalling failure and creep deformation. This talk discusses the development and application of ultrahigh temperature in situ transmission electron microscopy characterization based mechanical testing using localized laser heating. The approach is applied to characterizing interfacial strain at individual grain boundaries. Interestingly, bicrystal experiments suggest interfacial strain tends to be governed by nucleation rate limited kinetics, rather than diffusional kinetics, up to relatively large stresses in many materials, i.e. 100’s of MPa. These data are utilized to formulate a new model for grain boundary creep. The phenomenon is also observed at metal-oxide interfaces, and implications for environmental barrier coatings will be discussed.