Table 1. Comparative list of the criteria for PES from Wunder (2005) and of criteria and considerations from this paper with justifications for the changes made in this paper. This paper Wunder Justification for differences between definitions Criteria conditionality conditionality Methodological core of both definitions that incorporates monitoring and the definition of the service goal. positive incentives Ideological core to both definitions and implicit in Wunder’s; is also implicit in the use of the term “payment” as opposed to fines, although positive incentives are not limited to monetary transfers. Considerations additionality Reflective of the social or ecological goals of the intervention and allows wider impact to be measured. Discussed by Wunder but not a criterion. institutional context Practical implementation issues that will vary among PES interventions, implicit in Wunder’s discussion of buyers, service providers, voluntary and PES-like interventions. voluntary Falls within a consideration of institutional context, as the voluntary criterion may be met to varying degrees within a conditional positive incentive. well-defined service Implicit in the concept of conditionality. buyer Implicit in the idea of a transfer and fits better as a component of the institutional context. service provider Same as for buyers.