Increased participation of non-governmental organizations (NGOs) in international lawmaking is hardly questioned in scholarship. NGOs use different means to affect international lawmaking, including acting as complainants and providing legal advice to the petitioners. However, in what particular way do NGOs influence international lawmaking? The dissertation answers this question by examining how NGOs utilize the procedural instrument of amicus curiae intervention before international tribunals. The dissertation shows that amicus curiae interventions by NGOs have become commonplace in international adjudication. In general, amicus curiae participation procedure serves as a legitimacy-enhancing mechanism for international tribunals. Scholars agree that in order to maintain legitimacy the international tribunals must stay cognizant of the values and preferences of stakeholders. Amicus intervention procedure is one of the mechanisms through which tribunals gather information about the values and attitudes of constituencies and communities subject to their lawmaking. Moreover, NGOs as amicus interveners act as "global law entrepreneurs": they provide the tribunals with information as well as continuously support international lawmaking and advocate for its expansion.