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. #xE2;#xAC;#xA6; It may serve for teaching to higher graduate students and for researchers in the field of nonlinear PDE#xE2;#xAC;"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 #xE2;#xAC;#xA6; . 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. #xE2;#xAC;#xA6; 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).