COST is a European programme promoting co-operation in science and technology. Today there are over 200 active COST networks (or so-called Actions) operating in 32 countries. Of these Actions, 13 are in the materials sector. This book contains the proceedings of COST 525: "Advanced Electroceramics - Grain Boundary Engineering", the main objective of which is to understand the role played by grain boundaries in controlling the manufacture, microstructure and properties of electronic ceramics. The increase in knowledge should lead to the development of materials with enhanced properties, improved stability, reduced component costs, and the possibility of new opportunities for existing and developing ceramics.