<p>Halide-based solid electrolytes (HSEs) have garnered substantial interest for all-solid-state batteries (ASSBs) due to their wide electrochemical windows, moderate-to-high room-temperature ionic conductivity, and enhanced air stability over traditional sulfide and oxide-based SEs. This review consolidates recent advances in HSEs, focusing on the link between structure, compositions, and materials properties that influence the transport of lithium-ion (Li-ion) and post-lithium-ion (P-Li-ion) and their stability at the interface. Based on the chemistry of their central metal, HSEs are divided into five classes; key factors influencing ionic conductivity are examined. Nevertheless, despite these benefits, many challenges remain, including interfacial instability, the trade-off between ionic conductivity and electrochemical stability, mechanical challenges, and material costs. The main synthesis methods, mechanochemical, co-melting, and wet-chemical, are investigated for phase formation, scalability, and defect control. The link between synthesis, microstructure, and device-level performance metrics, including critical current density, area-specific resistance, and cycle life, is examined. The strategies, involving bilayer and dual-electrolyte design as well as interface engineering, are analyzed to reduce interfacial resistance and dendrite growth. The applications of HSE in Li-ion and P-Li-ion systems are examined. This review offers a detailed framework and delineates potential research paths to advance scalable, high-performance HSEs for next-generation ASSBs. </p>