The Sourcebook consists of a series of briefs on selected topics. The briefs include practical examples and graphics to explain various concepts. The aim is to make each brief a stand-alone document so that practitioners can directly access a particular section without necessarily reading all the earlier sections. At the end of many briefs, further relevant readings are suggested. It will be helpful, however, to start with Brief 1, 'Basic Principles of PES,' which provides an overview of the concept of payments for environmental services.