The international community has committed itself to achieve, by 2010, a significant reduction of the current rate of biodiversity loss at the global, regional, and national levels. Yet, despite growing awareness, and major efforts in all countries, the latest evidence indicates that biodiversity continues to be lost at a terrifying pace, resulting in what some call the greatest mass extinction since dinosaurs roamed the planet, 65 million years ago. A range of methods have been developed to value ecosystems, and the services they provide, as well as the costs of conservation. The methods available are increasingly sensitive, and robust, but they are often incorrectly used. One reason is poor understanding of the purposes of valuation and what questions it can, or cannot, answer. As a result, decision makers may get misleading guidance on the value of ecosystems, and their conservation. In this context, the Bank, IUCN-The World Conservation Union, and the Nature Conservancy have worked together to clarify the aims and uses of economic valuation, focusing on the types of questions that valuation can answer, and the type of valuation that is best suited to each purpose. 'How Much is an Ecosystem Worth?' is the result of that cooperation. It aims to provide guidance on how economic valuation can be used to address specific, policy-relevant questions about nature conservation.