From the reviews: "The book under review presents several topics of the regularity theory for nonlinear elliptic equations and systems which have been developed in recent years. ... It may serve for teaching to higher graduate students and for researchers in the field of nonlinear PDE's." (Joachim Naumann, Zentralblatt MATH, Vol. 1055, 2005) "It is a pleasure to review this new book by Bensoussan and Frehse; indeed this work covers remarkable, finely selected, updated material ... . this book is a very interesting and useful source of material for those interested in the latest developments of regularity theory of solutions of nonlinear elliptic systems. ... the book is an essential companion to the standard monographs already available and it is bound to become a standard and inevitable item in any list of references on the subject." (Giuseppe Mingione, Mathematical Reviews, 2004 a)