The book develops principles of intergenerational ethics in four main areas. How good a life are we required to leave future people? When is it permissible for an individual to procreate? Should we try to ensure as many people as possible live in the future or allow humans to become extinct? How does the fact that we do not always know how our actions will affect future people change what we owe them? The book answers these questions by using the contractualist method to develop general moral principles in these areas, and then applying those principles to real-world, concrete situations that individuals and states face every day.