Often with only the limited instruments provided in environmental treaties to promote compliance, the compliance committees under18 multilateral environmental treaties (MEAs) over a period of 40 years have looked into 321 individual cases of non-compliance and have issued 76 reviews of national reporting by hundreds of states. This thesis assesses the impact of these compliance committees operating under MEAs on national legal orders. The systematization of the practice of the compliance committees reveals that of the 18 committees researched, only 12 employed practices that allow for a meaningful impact assessment. These 12 committees, depending on the degree of their impact, are of three categories: Category (A) - high impact compliance committees; Category (B) - intermediate impact compliance committees; Category (C) - low impact compliance committees. The work of these committees has ensured, amongst others, (1) more distributional and procedural environmental justice, (2) the essential decrease in the consumption of CFCs by developing countries, (3) improvement of reporting obligations, (4) more reasonable use of marine living resources in Antarctic region, (5) better reporting on transboundary movement of hazardous waste, and so on. The empirical findings illustrate that the influence of the compliance committees is embedded in the overall success of the respective treaties. Upholding the fairness school of Thomas Franck, the empirics of impact assessment suggests that the compliance of states with international environmental law is directly linked to the emergence of shared beliefs about the fairness of any treaties concluded. Absent a genuine agreement on shared objectives across the continents (e.g. the Kyoto Protocol, the Basel Convention, and the Mediterranean/Barcelona Convention), the influence of the compliance committees of the respective treaties has been minimal or almost none.