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.