Scholars have made great advances in modeling and mapping ecosystem services, and in assigning economic values to these services. This modeling and valuation scholarship is often disconnected from evidence about how actual conservation programs have affected ecosystem services, however. Without a stronger evidence base, decision makers find it difficult to use the insights from modeling and valuation to design effective policies and programs. To strengthen the evidence base, scholars have advanced our understanding of the causal pathways between conservation actions and environmental outcomes, but their studies measure impacts on imperfect proxies for ecosystem services (e.g., avoidance of deforestation). To be useful to decision makers, these impacts must be translated into changes in ecosystem services and values. To illustrate how this translation can be done, we estimated the impacts of protected areas in Brazil, Costa Rica, Indonesia, and Thailand on carbon storage in forests. We found that protected areas in these conservation hotspots have stored at least an additional 1,000 Mt of CO 2 in forests and have delivered ecosystem services worth at least $5 billion. This aggregate impact masks important spatial heterogeneity, however. Moreover, the spatial variability of impacts on carbon storage is the not the same as the spatial variability of impacts on avoided deforestation. These findings lead us to describe a research program that extends our framework to study other ecosystem services, to uncover the mechanisms by which ecosystem protection benefits humans, and to tie cost-benefit analyses to conservation planning so that we can obtain the greatest return on scarce conservation funds.T o inform decision makers about how they can best allocate resources to maintain and enhance ecosystem services, scholars need to develop a better understanding of how policy interventions actually affect the supply of ecosystem services. In the last two decades, scholars have made important advances in defining, measuring, and valuing ecosystem services across time and space (1-7). These measures and values have in turn been used as conservation planning and resource management decision tools (8-11). The degree to which policies and programs have affected these services and values in the past remains poorly understood, however (12, 13). Without an evidence base for the impacts of real policies and programs on ecosystem services, the insights from modeling and valuation are not as informative to decision makers as they could be.A separate and more recent literature focuses on estimating the impacts of conservation policies and programs on environmental and social outcomes (reviewed in refs. 14 and 15). Scholars working in this impact evaluation literature strive to eliminate rival explanations for the empirical patterns observed, so that any correlations between programs and outcomes can be confidently attributed to causal relationships; however, these studies do not estimate impacts on ecosystem services, but rather estima...