This book grew out of the fourth Special Year at DIMACS, which was devoted to the subject of combinatorial optimization. During the special year, a number of workshops, small and large, dealt with various aspects of this theme. Organizers of the workshops and selected participants were asked to write surveys about the hottest results and ideas in their fields. Therefore, this book is not a set of conference proceedings but rather a carefully refereed collection of invited survey articles written by outstanding researchers. Aimed at researchers in discrete mathematics, operations research, and the theory of computing, this book offers an in-depth look at many topics not treated in textbooks.