All-solid-state lithium batteries (ASSLBs), which use solid electrolytes instead of liquid ones, have become a hot research topic due to their high energy and power density, ability to solve battery safety issues, and capabilities to fulfill the increasing demand for energy storage in electric vehicles and smart grid applications. Garnet-type solid electrolytes have attracted considerable interest as they meet all the properties of an ideal solid electrolyte for ASSLBs. The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) has excellent environmental stability; experiments and computational analyses showed that this solid electrolyte has a high lithium (Li) ionic conductivity (10⁻⁴–10⁻³ S·cm⁻¹), an electrochemical window as wide as 6 V, stability against Li metal anode, and compatibility with most of the cathode materials. In this review, we present the fundamentals of garnet-type solid electrolytes, preparation methods, air stability, some strategies for improving the conductivity based on experimental and computational results, interfacial issues, and finally applications and challenges for future developments of LLZO solid electrolytes for ASSLBs.