This volume is organized around two broad themes: graph theory with path-breaking contributions on the theory of flows in networks and project scheduling; and multiple criteria decision aiding with the invention of the family of ELECTRE methods and methodological contribution to decision-aiding which lead to the creation of Multi-Criteria Decision Analysis (MCDA). Part one contains papers by Jacques Lesourne, and Dominique de Werra and Pierre Hansen related to the early career of Bernard Roy when he developed many techniques and concepts in graph theory in order to cope with complex real-world problems. Part two of the book is devoted to philosophy and epistemology of decision-aiding with contributions from Valerie Belton and Jacques Pictet and Jean-Luis Genard and Marc Pirlot. Part three includes contributions based on theory and methodology of multi-criteria decision-aiding based on a general framework for conjoint measurement that allows intrasitive preferences. Part five groups applications of multi-criteria decision-aiding, and offers a wide variety of application problems. Finally, Part six includes contributions on multi-objective mathematical programming from Jacques Teghem, Walter Habenicht and Pekka Korhonen.