Bluefin tuna is one of the most commercially important fish species not just for Croatia but for all Mediterranean countries. Due to it trans-boundary character its management is arranged through International Commission for Conservation of Atlantic Tuna (ICCAT). But due to increased marked demand, overfishing and a rising global tendency of NGO participation, various types of non-governmental NGOs began to take interest in this issue. Therefore, this book discusses and analyzes ways of NGOs participation in ICCAT considering a specific species the Bluefin tuna. Participation of three kinds of NGOs was analyzed at two levels: global and national. Participation of international NGOs is explained through their engagement in enhancing the knowledge base (science, policy and law), advocacy and lobbying, membership in international delegations, contribution to compliance, ensuring the transparency, supporting international secretariats and border function of NGOs in international environmental governance. Due to limits of data the study of participation of Croatian national NGOs was based just on ENGOs. For that purpose, an overview of ENGOs work is given.