This book focuses on ion transport and storage around the interfaces in solid-state-ionics devices, especially for all-solid-state batteries. It covers materials research from fundamental model systems to practically important materials, advanced analysis methods, and computational & data science. This volume is intended for researchers in academia and industry who have studied or worked in this field. It is expected to provide readers with guidelines for interface and material design aimed at advancing all-solid-state batteries, as well as insights into the development of novel solid-state ionics devices.This book is a summary of the studies obtained in the Grant-in-Aid for Scientific Research on Innovative Areas "Interface Ionics” founded by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) during 2019-2023.