Kenneth Arrow and Gerard Debreu have throughout their careers continuously produced ideas at the very frontier of economics. Together, they have made unparalleled contributions on the properties of general equilibrium systems in economics, the study of collective choice and welfare economics. The editors have shown their usual rigor in selecting those papers which, in their view, have made the most important contributions in their particular areas of expertise. This volume will be an essential source of reference for students, researchers and practitioners alike.