Payment for Ecosystem Services (PES) is becoming increasingly popular as a way to manage ecosystems using economic incentives. The environmental economics approach to PES tries to force ecosystem services into the market model, with an emphasis on efficiency. The ecological economics approach, in contrast, seeks to adapt economic institutions to the physical characteristics of ecosystem services prioritizing ecological sustainability and just distribution and requiring a transdisciplinary approach. This paper summarizes the results of a participatory "atelier" workshop held in Costa Rica. We developed a set of principles (the Heredia Declaration) for PES systems and report on evolving initiatives in several countries. We discuss how the distinction between ecosystem goods (which are stock-flow resources) and ecosystem services (which are fund-service resources) and the physical characteristics of the fund-services affect the appropriate institutional form for PES. We conclude that PES systems represent an important way to effectively manage fund-service resources as public goods, and that this represents a significant departure from conventional market institutions.