During the last decade, the notions of environmental services and ecosystem services (ES) experienced a rapid development of attention from scholars and development actors. These notions have been mobilized to develop new instruments to face environmental problems such as deforestation: the Payments for Environmental Services (PES). Nevertheless, the integration of these concepts in public policy is still limited. In this regard, Costa Rica is considered as a pioneer since it has integrated the ES notion in its forest law since 1996, setting the ground for the implementation of the national Program of PES (PPES). Although the nature and efficiency of the PPES has been largely discussed, the policy process has been poorly documented. In this article, we aim at understanding how and why the concept of ES, then poorly known at international level, was integrated at this specific moment in the Costa Rican forestry law #7575. Based on a review of existing documents and interviews of actors involved directly or indirectly in the policy process, we analyze the formulation and adoption of the forestry law #7575. We show that the introduction of the notion of ES resulted from an original policy windows, which derived from 4 factors: 1) the pre-existence of an identified problem of deforestation, 2) the existence, for more than two decades, of instruments supporting forestry sector and of interest groups structured in the forestry sector, 3) the development of flows of ideas inside Costa Rican elites inserted in international networks, 4) the presence of policy entrepreneurs that were able to take advantage of the national and international context to build a compromise leading to the new Costa Rican forestry policy.