The book "Oxygen Ion and Mixed Conductors and their Technological Applications" provides a comprehensive analysis of the advancements in the development of oxygen ion and mixed conductors. These materials are crucial for innovative applications in areas such as gas sensors, fuel cells, oxygen-permeable membranes, oxygen pumps, and electrolyzers. Despite their promising properties, the commercialization and application of these technologies face challenges, including material stability, compatibility issues, and high production costs. The authors, a group of experts, highlight in their contributions the fundamental aspects that are important for understanding the electrochemical and interfacial processes. The book is based on the findings of a NATO Advanced Study Institute held in 1997 in Erice, Sicily, and offers an integrated and synthetic approach to the topic, supported by numerous examples and case studies.