The participation of environmental NGOs on the international plane has largely been discussed in the contexts of treaty negotiation and adjudication. However, their roles within non-adjudicative implementation and compliance processes—a notable feature of international environmental law—remain underexplored. This seminar discusses environmental NGOs’ compliance-enhancing roles, locating its analysis in two types of civil society participation mechanisms that modern free trade agreements have increasingly incorporated to monitor compliance with environmental provisions. Based on extensive empirical data collected from semi-structured interviews and documentary analysis, it maps environmental NGOs’ compliance-enhancing roles as informants, interpreters, and response generators and examines their breadth and limits.