This article is about the concept of optimum population and consumption. Even though it is primarily concerned with foundational issues, the various ideas that have been discussed in the literature are tested in the context of economic models with limited resources. It is argued that, broadley speaking, existing theories of optimum population and consumption are variants of average and, what is often termed classical, utilitarianism, respectively. Both classes of theories, are shown to have deficiencies, in great measure because of their reliance on a defective concept of personhood. It is also argued that contractual theories are of little use, because potential people (as opposed to future people) cannot be parties to any contract. A generation-relative ethics is developed and is put to work in an overlapping generations model. It is shown that generation-relative ethics, even when it is a variant of classical utilitarianism, can prescribe considerably lower population than classical utilitarianism.