This brief volume offers a focused and accessible overview of electrode-electrolyte interphases in solid-state batteries, emphasizing their formation, composition, and critical influence on battery performance and longevity. It examines the thermodynamic, kinetic, and mechanistic aspects of interphase development across different classes of solid electrolytes, while exploring how operating conditions such as temperature, pressure, and electro-chemo-mechanical coupling drive their evolution under realistic environments. The book highlights the dual role of these interphases as both enablers and constraints, discussing their impact on ion transport, interfacial stability, resistance growth, and failure mechanisms, including dendrite formation. It also presents state-of-the-art nanoscale characterization techniques including in-situ, operando, ex-situ and computational approaches used to investigate interphase structure, dynamics, and transport phenomena. Complemented by an overview of interfacial engineering strategies aimed at improving stability and mitigating degradation, this concise volume is designed to support researchers and students in understanding, designing, and controlling interphases for high-performance, durable solid-state energy storage systems.