The concept of ecosystem services is proliferating rapidly in research and policy worldwide and holds strong appeal as a bridging concept between the natural world and society. In applying the concept, we should remain alert to some ‘health warnings’ from previous experience in environmental economics. (1) The ecosystem services concept involves a narrowing of focus onto the money value of ecosystems. Researchers and practitioners should reflect on past efforts in integrative benefits analysis. (2) There can be a conceptual disconnection of value from function, particularly when there is a reliance on benefits transfer, yet the value of ecosystem services is conditional on a well-functioning whole ecosystem. (3) The problem of data paucity is stark. While inadequate data and the resulting uncertainty are ubiquitous problems in environmental decision-making, the powerful simplicity of one number—the estimated monetary value of a given ecosystem service—may mean that more nuanced precautionary considerations that normally apply in uncertain contexts are bypassed. New challenges arise because values (determined in the past) are applied to a changing and already resource-depleted world. Here I highlight the non-stationary nature of trends, the extent to which models are trusted and its role in uncertainty and risk, and the need for in-depth interdisciplinary study. Some organisations have already entered into these meta-debates; examples are the joint Environment Agency/Quantifying and Understanding the Earth System (EA/QUEST) project on uplands and the Economic and Social Research Council-Natural Environment Research Council (ESRC-NERC) interdisciplinary workshop series in the UK, and, internationally, the Ecosystem Services Partnership. It is important that this debate is maintained and extended.