The importance of non-governmental organisations (NGOs) in international environmental co-operation has increased tremendously over the last decades. Accordingly, the participation of non-governmental actors has become a prominent subject for research, especially in the legal and social sciences. However, limited effort has been spent at systematically analysing the relationship between the legal basis and the practical influence of NGOs in different areas of international environmental co-operation. This study first laid a conceptual basis by reviewing existing definitions of NGOs and developing an own working definition, elaborating the functions NGOs perform in international environmental policy-making and examining various criteria that can serve to distinguish different types of NGOs.