Recent computation and experimental works have discovered a remarkable array of materials with superionic conductivities that rival or exceed the conductivity of conventional liquid electrolytes such as Li and Na conducting NASICON-type oxides, Li garnets, lithium-rich anti-perovskites, and thiophosphate materials. Unfortunately, the high ionic conductivity often is associated with a penalty of high impedance at interfaces between electrolyte and electrodes; this interfacial impedance can dominate the internal resistance in many systems. High impedance is found both in thiophosphate electrolytes in contact with oxide cathodes and during high temperature processing of oxide electrolytes cosintered with oxide cathodes. Because of the necessity of maintaining low interfacial impedance for high power applications, we have focused on understanding the reactions occurring at the various interfaces. This talk will focus on the development and implementation of computational thermodynamic methods to predict trends in reactivity and types of reaction products formed at the interfaces of a number of commonly investigated electrolyte systems, cathode materials, and cathode coatings. We use these results to identify combinations of materials that are thermodynamically stable over the range of processing and operating conditions experienced in high voltage all solid state battery systems.