We discuss ionic transport in fast lithium and sodium ion conductors, which receive renewed attention as solid-state electrolytes for all-solid-state batteries. Because transport of ions across interfaces can differ strongly from that in the bulk, the DC conductivity relevant for battery operation cannot be measured directly by impedance spectroscopy. We discuss models that accurately describe the impedance spectra of prototypical solid-state electrolyte materials including the effect of grain boundaries and ion-ion correlations.