Domain boundary engineering - recent progress and many open questions

Some domain boundaries contain functionalities which do not exist in the bulk. Typical examples are (super-) conducting twin walls in WO3, highly conducting walls in BiFeO3 and in structural interfaces between SrTiO3 and LaAlO3, and ferroelectric walls in CaTiO3. The emerging field of Domain Boundary Engineering endeavors to generate such functional interfaces in a multitude of materials for applications in device materials. Some of the recent successes are reviewed together with suggestions for further research.