Extensive efforts have been made to improve the Li-ionic conductivity of solid electrolytes (SE) for developing promising all-solid-state Li-based batteries (ASSB). Recent studies suggest that minimizing the existing interface problems is even more important than maximizing the conductivity of SE. Interfaces are essential in ASSB, and their properties significantly influence the battery performance. Interface problems, arising from both physical and (electro)chemical material properties, can significantly inhibit the transport of electrons and Li-ions in ASSB. Consequently, interface problems may result in interlayer formation, high impedances, immobilization of moveable Li-ions, loss of active host sites available to accommodate Li-ions, and Li-dendrite formation, all causing significant storage capacity losses and ultimately battery failures. The characteristic differences of interfaces between liquid- and solid-type Li-based batteries are presented here. Interface types, interlayer origin, physical and chemical structures, properties, time evolution, complex interrelations between various factors, and promising interfacial tailoring approaches are reviewed. Furthermore, recent advances in the interface-sensitive or depth-resolved analytical tools that can provide mechanistic insights into the interlayer formation and strategies to tailor the interlayer formation, composition, and properties are discussed.