Healthy ecosystems provide many environmental services (ES) such as carbon mitigation, water protection, biodiversity, and scenic beauty that are indispensable for the well-being of humans. Payments for Environmental Services (PES) programmes provide financial incentives for the conservation, sustainable management, and restoration of the ecosystems that ensure these services. Although PES programmes are growing in popularity within the sustainable development community, they remain a relatively new approach to environmental conservation and economic development. To further our understanding of how PES programmes work, this project presents a case study of Costa Rica’s fifteen-year old PES programme, known locally as Pagos de Servicios Ambientales (PSA), with a focus on institutional decision making. The Institutional Analysis and Development (IAD) framework is applied in order to identity the actors, rules, and activities governing the programme and affecting its outcomes. Rules and activities are examined at the IAD’s four levels of analysis (operational-choice, collective-choice, constitutional-choice, and metaconstitutional-choice). Results suggest that decisions made at the two most upper tiers (constitutional-choice and metaconstitutional-choice) have had a large role in influencing the operational-choice level activities that have a direct effect on the programme’s outcomes.