The Water Framework Directive (WFD) is perhaps the most ambitious piece of environmental legislation in the history of the European Union. The Directive consolidates existing water-related legislation and has the stated objective of delivering good status (GS) for Europe’s surface waters and groundwaters. But meeting GS is cost dependent, and in some water bodies pollution abatements costs may be high or judged as disproportionate. The exact definition and assessment of disproportionate costs is central for the justification of time-frame derogations and/or lowering the environmental objectives (standards) for compliance at a water body. Official guidance is somewhat discretionary about the interpretation of disproportionate costs. Building on basic cost-benefit theory, this paper attempts to clarify the meaning of disproportionate cost to non-economists, and to convey a consistent interpretation that should underlie the development of a practical derogation decision making across all member states