The World Commission on Environment and Development provided the conceptual foundation for sustainability with its statement, “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own need,” and this is often shortened to notion of non-declining social welfare. However, such a concept of sustainability is only as useful as our ability to measure it. Leading organizations like the World Bank and UNEP attempt to reform national accounting and to operationalize sustainability as non-declining comprehensive wealth as an approximation to non-declining social welfare. In this talk I will develop formal theory for using changes in wealth as an approximation for changes in social welfare. I will highlight some of the challenges with this approximation approach, especially in the multi-dimensional case. I will then suggest that in some cases the approximation may be unnecessary, so long as we pay careful attention to measuring the prices for capital assets, particularly natural and human capital. This is because wealth is the sum of price of assets times quantity of assets. However, when measuring natural (and human) capital special attention must be paid to adaptive ecological dynamics and human behaviors. These are not areas often considered in valuing assets. I will conclude by making the case that, with care, the non-declining wealth approach to sustainability provides a unifying framework for research on human-environmental systems that elevates the policy relevance of this research agenda.