Over the past decade there has been a substantial increase in the use of market based and voluntary financial instruments to address environmental issues. Using price premiums and/or financial incentives, these instruments try to achieve desirable land use practices and sustainable resource management. In recent years PES (Payment for Ecosystem Services) emerged as such an instrument with high potential. Roughly speaking, PES aims to issue incentives and compensations for extra costs induced to ecosystem owners and managers (e.g. farmers, water companies, etc.) for maintaining a stable or increased supply of ecosystem services. Although this seems a quite straightforward approach, much confusion arose about what PES really is, and how it can be defined. We explore why it is important to come to a single definition of PES for Flanders (Belgium) and identify in which setting PES would be an efficient and desirable alternative to current policy instruments. Additionally we compiled a series of 20 key conditions and success factors for PES instruments based on a thorough review of international (practical) experiences and state of the art literature. Based on these success factors, we critically examined and evaluated a series of financial, “PES-like”, instruments currently used in Flanders (e.g. agri-environment measures). For that purpose 3 case studies were conducted in the areas of Gent (Gentse Kanaalzone), Leuven (Doode Bemde) and Sint-Truiden (Melsterbeek) and focussed on a series of financial instruments (mainly subsidies) and their impact on ecosystem services and stakeholders. The ISEP project resulted in a guideline for the identification of ESD stakeholders and an opportunity analysis to manage and enhance additional ecosystem services with current financial instruments. We concluded that some existing financial instruments show great potential to mainstream a PES approach, which could greatly improve their ecosystem service output. Also participation to voluntary financial instruments can be ...