The notion of ecosystem services (ES) has been rising during the last decade. It has led to new generation of mechanisms to cope with environmental issue in various countries. Costa Rica was pioneer in operationalize the concept though a specific policy instrument: the Payment for Environmental Services program (PESP). Based on literature review and interviews of stakeholders, we analyse the policy process that have led to the emergence of the PSEP. We combine (i) the sequential approach of policy cycle to identify and describe the different phase of the emergence of Costa Rica PESP from the problem setting phase to the decision phase institutionalizing the program through the Costa Rican forestry law of 1996, and (ii) cognitive approach of policy process analysis to identify the internal and external factors that have led to the emergence of this program. We show that, in the case of Costa Rican PESP, the policy steps were rapids from the agenda setting to the decision due to external constraints and internal compromises between leading actors in a context of dominance of one group of interest. The emergence was facilitated by an early problem setting on deforestation and the existence of forestry incentive instruments as well as a consolidation of forestry organisations and institutions. The governance of the program reflected the existing balance of power between interest groups of different sectors (forestry, agricultural, conservation) involved in forest areas management. (Texte intégral)