This book addresses the issues of intergenerational equity, intertemporal efficiency, and collective rationality of social decision-making. How should the welfare of future generations be treated vis-a-vis the welfare of present generation? How should we design and implement social security reforms? How can we formalize the notion of sustainable economic development, and what are the conditions that warrant the sustainability of development path? In the context of the problem of global warming, where a large part of those who are responsible for causing the problem do not exist anymore, and a large part of those who will be most severely affected by the problem do not yet exist, why and how should the present generation assume responsibility to take actions against this problem? Contributions come from distinguished economists who specialize in this area, and are tied by the common thread of taking a welfare-theoretic approach to the problem. Introduction; J. Roemer & K. Suzumura -- PART 1: EQUITY AMONG OVERLAPPING GENERATIONS -- Pension Contributions and Capital Accumulation; T. Ihori -- Equity and Efficiency in Overlapping Generations Economies; T. Shinotsuka, K. Suga, K. Suzumura & K. Tadenuma -- Social Security Pensions and Interregional Equity: The Japanese Case; N. Takayama -- PART 2: RANKING INFINITE UTILITY STREAMS -- A New Equity Condition for Infinite Utility Streams and the Possibility of Being Paretian; G.B. Asheim, T. Mitra & B. Tungodden -- Possibility Theorems for Equitably Aggregating Infinite Utility Systems; K. Basu & T. Mitra -- On the Existence of Paretian Social Welfare Quasi-orderings for Infinite Utility Streams with Extended Anonymity; T. Mitra & K. Basu -- Pareto Principle and Intergenerational Equity: Immediate Impatience, Universal Indifference and Impossibility; Y. Xu -- PART 3: INTERGENERATIONAL EVALUATIONS -- Formal Welfarism and Intergenerational Equity; C.d'Aspremont -- Intertemporal Social Evaluation; C. Blackorby, W. Bossert & D. Donaldson -- Intergenerational Fairness; M. Fleurbaey -- Person-Affecting Paretian Egaliterianism with Variable Population Size; B. Tungodden & P. Vallentyne -- PART 4: SUSTAINABILITY AND HUMAN DEVELOPMENT -- Intergenerational Justice and Sustainability under the Leximin Ethic; J. Roemer -- Intergenerational Justice, International Relations, and Sustainability; J. Roemer & R. Veneziani -- Intergenerational Equity and Human Development; J. Silvestre -- PART 5: LONG-RUN ISSUES OF INTERGENERATIONAL EQUITY -- Toward a Just Savings Principle; N. Van Long -- Normative Approaches to the Issues of Global Warming: Responsibility and Compensation; K. Suzumura & K. Tadenuma -- Fundamental Incompatibility Between Economic Efficiency, Intergenerational Equity and Sustainability; N. Yoshihara -- Index.