Solid lithium-ion-conducting material is the key component in the fabrication of next-generation all solid state lithium ion batteries (LIBs) which would exhibit superior safety and performance compared with the currently widely used ones that resort to essentially inflammable and volatile organic solvents.To date,great efforts have been made in developing solid conductors with high lithium ion conductivity,such as polymers and inorganic materials.Rare earths play a very important role in this area and have attracted extensive interest since the recent decades for their unique properties in the realm of solidstate inorganic lithium-ion-conducting electrolyte materials.In this introduction,we focus on the role of rare earths in solid conductors for lithium ion,especially in a few most studied systems such as perovskites,garnets,silicates,borohydride and the recently reported halides in which rare earths act as a key framing component.Besides,the effect of rare earths as dopants is also discussed in some recently studied systems.Valence,coordination,and size are the most important factors that influence the crystal structure and property of these lithium ion conductors.The aim of this review is to highlight the great potentials of these unique elements of rare earths,and to help improve the performance of existing materials and explore new applications in the development of new LIBs with high performances.