About forty colleagues from France and abroad were pleased to come to Caen from 10th to 12th June 2009 to thank and honor Maurice Salles for the outstanding work he accomplished in the field. The meeting, entitled "New Developments in Social Choice and Welfare Theories--Conferences in Honor of Maurice Salles" was the annual conference of the ADRES (Association pour le Développement de la Recherche en Economie et Statistiques). Preliminary versions of most of the papers in this special issue of Social Choice and Welfare were presented at the conference. The organizers were pleased to host Amartya Sen as a plenary speaker, and thank him for letting them publish his speech. The other nineteen articles collected here examine five aspects of modern social choice theory: Preferences and Aggregation, Voting Rules and Ranking Rules, Applications of Cooperative Game Theory, Implementing Fairness in Economic Environments, and Inequality, Poverty and Opportunities.