Solid-state batteries offer improved energy density and better safety characteristics compared to lithium-ion batteries, but interfacial evolution and chemo-mechanical degradation can limit performance. In this talk, I will present my group’s recent work using in situ and operando experiments to understand nanoscale-to-macroscale evolution of materials and interfaces in solid-state batteries. Operando X-ray tomography experiments of solid-state batteries with lithium metal anodes are shown to enable real-time quantification of interfacial contact loss between lithium metal and a solid electrolyte during cycling, which is found to cause current constriction and cell failure. Deposition/stripping behavior in lithium-based batteries is also investigated in real time. Further studies reveal chemical transformations at the interface between lithium and solid electrolytes that lead to the growth of an “interphase”, which is shown to induce large stresses and to cause fracture of the solid electrolyte. Finally, reaction mechanisms in alloy-based anode materials within solid-state batteries are probed with in situ stack pressure measurements. Together, these findings show the controlling impact of the evolution of the structure, chemistry, and mechanics of the materials within solid-state batteries, with in situ and operando experiments being critical for understanding these processes.