This chapter traces the historical and contingent nature of the contemporary dominance of economic forms of valuation in performing value. It reviews the diverse ways in which traditional approaches to valuation are found inadequate from a conceptual and practical standpoint. The chapter focuses on the production of ‘value’ through the creation of new ‘value entities’ that can be accounted for, costed and circulated in monetised and financialised forms, such as within a market in which they have a price. The development of environmentalism in the 1970s generated narratives that transformed the separation between the aggregating approach of economics and the pluralist assessment of incommensurable phenomena in environmental impact assessment. The economic imagination of externalities is one that revolves around the categories of costs and benefits: if something is left out of a market it is either a cost or a benefit to some party outside that market.