Legal and economic problems relating to climate-change mitigation transgress national boundaries and thus are at odds with the Westphalian system of territorially defined allocations of powers and responsibilities of government. This problem, of course, is not new and has significantly shaped the contours and concepts of international environmental law. States and the international community have shown considerable imagination and engaged in innovative legal engineering to cope with transnational issues. They have crafted emerging principles, rules, and monitoring mechanisms designed to strike a balance between two conflicting requirements: on the one hand, there is a pressing need to put an end to, or at least to slow down, the deterioration of the environment as well as forestalling new damage. On the other hand, there is a necessity for a realistic appraisal of the existing structures and the social and economic costs involved in this process both for developed states and even more so for developing countries. In addition, all of the above is undertaken within a framework of fragmented jurisdictions among states, which adds to the complexity of the task. For such reasons, progress at the legal level has been less conspicuous than one would have expected or desired. But progress has been made. The environment is no longer conceived of from a state-sovereignty-oriented perspective, as an asset that may belong to each state and in whose protection only the state concerned may be legally and practically interested.