International environmental law, also known as ‘green international law’, not only presents numerous challenges but also offers significant opportunities to protect the global environment. In this chapter, the term green international law is used synonymously with international environmental law, referring to the body of international legal norms, treaties, and soft law instruments that regulate the protection of the global environment. It encompasses obligations on states to prevent environmental harm, mechanisms for transboundary cooperation, and emerging principles such as sustainable development and intergenerational equity. This constantly evolving field addresses issues such as climate change, biodiversity loss, and transboundary pollution. Its objective is to promote sustainable development, preserve natural resources, and prevent environmental degradation. In our study, we will examine the role of international environmental law as a branch of international law in the application and enforcement of law on states. This is particularly relevant given the leading role now played by international non-governmental organisations and individuals. We will also consider how this influence has become effective in the application of international environmental law, and consequently international law in general. This study also aims to analyse the relationship between soft and hard law, and why states might adopt soft law more frequently to regulate environmental issues. This study also aims to analyse the relationship between soft and hard law, and why states might adopt soft law more frequently to regulate environmental issues.