Whether from public or private sources, whether regional or global, on a regular basis numerous and lengthy reports bear witness to the continued deterioration of the state of the environment. At the beginning of the 21st century, environmental changes are of great concern and in several respects carry the risk of being irreversible. The latest environmental assessment issued by UNEP, referred to as “GEO3” for Global Environmental Outlook, warns of continued deforestation and loss of biodiversity at an unprecedented pace in the history of humankind. According to the report, “the increasing pace of change and degree of interaction between regions and issues has made it more difficult than ever to look into the future with confidence” (UNEP, 2002). The World Bank's report on World Development in 2003 notes that the next fifty years could see world population increase by 50% reaching 9 billion people and a fourfold increase in gross domestic product reaching 140 billion dollars. This kind of growth risks, by its unreasonable expansion and unequal distribution, provoking social and environmental tensions threatening development efforts and living conditions. For more than thirty years, legal instruments have been used to protect the environment, and specifically international law when the stakes involve a strong transnational aspect. International environmental law has experienced remarkable growth both in quantity and in scope. Following a “frenetic” normative phase, during which the objective was to build a body of rules and little attention was paid to implementation, a relative ineffectiveness of the legal instruments adopted was observed. At the beginning of the 90s, legal scholars and practitioners started thinking about the reasons for such relative ineffectiveness and possible solutions. Lawyers and experts in international relations follow the same trend: after having focused mostly on the terms of adoption and the substance of international regimes, they are now focusing on implementation (RISSE, 2000). Until then, a rationalistic bias had lead to the hasty conclusion that: (1) governments make commitments only after having determined that these are in their interests, (2) and therefore they generally implement treaties and comply with their commitments and that (3) if they do not do so, sanctions will be applied to “punish” non-compliance as well as to deter other potential breaches. Reality is totally different and a lot more subtle, in particular in the environmental field where many diverse reasons can motivate States to make commitments, and where they sometimes make them without even intending to implement the commitments, or they otherwise attempt to implement their commitments but lack the necessary means (BROWN WEISS & JACOBSON, 1998). The issue of effectiveness has gradually become a major field of research in economics and international relations, and in international law (SPRINZ, 2000), giving rise to various analyses, from the most empirical to the most theoretical, with writers attempting to qualify or even quantify (CARSTEN & SPRINZ, 2000; SPRINZ, 2003) the degree of effectiveness of instruments and to explain the disparities observed. As part of a constantly evolving legal and institutional order, understanding these phenomena is a crucial preliminary requirement before attempting to strengthen the body of rules and instruments, and more broadly, improving the international “governance” of the environment. This is a burning issue. In spite of intense diplomatic activity, Heads of State and Governments which met during the Earth Summit Rio+5 in New York in June 1997 admitted they were “deeply concerned that the overall trends with respect to sustainable development [were] worse today than they were in 1992” and committed “[them]selves to ensuring that the next comprehensive review of Agenda 21 in the year 2002 demonstrates greater measurable progress in achieving sustainable development.” Five years later, the Johannesburg Declaration echoed this pessimistic assessment: “The global environment continues to suffer.” “Loss of biodiversity continues, fish stocks continue to be depleted, desertification claims more and more fertile land, the adverse effects of climate change are already evident, natural disasters are more frequent and more devastating, and developing countries more vulnerable, and air, water and marine pollution continue to rob millions of a decent life.” ... This study attempts first to describe and assess monitoring and implementation mechanisms (Section I), and second to put forward a few trails for improving and strengthening these mechanisms (Section II).