Despite growing recognition of the importance of environmental criteria, many investment decisions by development agencies are taken primarily on grounds of economics, and, specifically, on returns to investment. If natural resource considerations are to be allotted their rightful importance, there is no avoiding their conversion into money terms. Economic values are needed for analysis of soil and other land conservation projects, for estimating the loss to society caused by land degradation, and for national environmental accounting. Conventional economic methods undervalue natural resources; they may appear to come free, or to be priced at either their marginal or average values, ignoring the far higher price that would be paid if they became scarce. Above all, the practice of discounting, as employed in cost–benefit analysis, grossly underestimates future option values, the value of resources for use in the future. A consequence is that the economic losses caused by erosion, salinization, and other kinds of degradation are greatly undervalued. Economic methods as currently applied give equal weight to the needs of today's poor, but they steal resources, and thus welfare, from future generations. Assigning a value to land resources equal to their productive potential for at least 500 years, which virtually amounts to a sustainability constraint, would help to remedy this iniquitous situation.