The Handbook of Mathematical Economics aims to provide a definitive source, reference, and teaching supplement for the field of mathematical economics. It surveys, as of the late 1970's the state of the art of mathematical economics. This is a constantly developing field and all authors were invited to review and to appraise the current status and recent developments in their presentations. In addition to its use as a reference, it is intended that this Handbook will assist researchers and students working in one branch of mathematical economics to become acquainted with other branches of this field. Volume 3 covers Mathematical Approaches to Welfare Economics, including social choice theory, optimal taxation, and optimal growth, together with Mathematical Approaches to Economic Organization and Planning, including organization design and decentralization.