This volume of the EMS contains three contributions covering topics in the field of partial differential equations. All the authors are well-known researchers and they present their material as accessible surveys, enabling readers to find comprehensive coverage of results which are scattered throughout the literature. For this reason the book is a unique source of information. It forms part of a multi-volume subseries of the EMS devoted to partial differential equations and it will be very useful to graduate students and researchers in mathematics and theoretical physics as well as engineers who are interested in this subject.