Solid-state electrolytes are intensively being investigated as a critical component in future generation Li-metal battery systems, including their uses as the primary electrolyte in all-solid-state batteries, and as the protection layer for Li-metal in Li-air, Li-S, or aqueous batteries. While several solid electrolyte materials were developed recently that demonstrated sufficient conductivity, unexpectedly high resistance and cycling instability from grain boundaries and interfaces are often observed. The practical applications of solid-state electrolytes cannot be realized without a thorough understanding of these phenomena. This talk will cover our recent efforts in elucidating the fundamental origin of (1) interfacial resistivity between solid electrolytes and Lithium metal, and (2) preferential dendrite growth along grain boundaries, by using in situ and atomic-resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Interfacial phenomena among different solid electrolytes, e.g. Li_(7−3×)Al_xLa₃Zr₂O₁₂ (LLAZO), Li_1+xTi_2–xAl_x(PO4)₃ (LATP) and LIPON, will be compared and potential strategies to mitigate interfacial challenges will be proposed.