Imagine a family that inherits a beautiful old farmhouse. Each generation lives there and enjoys it, but they also know they must pass it on in good shape—otherwise, the ones who come later will be left with a crumbling ruin. Our planet is much like that farmhouse. We enjoy fresh air, clean water, forests, and minerals today, but if we use them up or wreck them, what will be left for our grandchildren? This chapter answers that question in a careful, economic way. We will look at what it means for a society to be sustainable across generations, how to think about fairness when some people are not yet born, and what tools we have—both in theory and in practice—to leave a world that is at least as livable as the one we inherited.