The excellent material presentation of this small book follows the tradition of the series. Jean Mawhin, SIAM Review ...I recommend Ghoussoub's book to anyone working on variational problems, in particular on LS-theory. The reader will find a systematic presentation and detailed exposition of useful techniques. T. Bartsch, Bulletin of the American Mathematical Society ...Each chapter ends with some historical notes and comments, which offer a very useful guide through the literature on the subjects mentioned. Maria Letizia Bertotti, Mathematical Reviews