Joint Implementation is one of three flexibility mechanisms established to allow developed countries to receive credit for greenhouse gas emissions reductions activities conducted in developed countries against net emissions reductions commitments undertaken in the Kyoto Protocol to the United Nations Framework Convention for Climate Change. Legitimacy, comprising a variety of procedural and substantive principles of international law, concerns questions about why law and its institutions should be respected. Joint Implementation has problems that can be classified as 1) mistargeting, 2) commodification and privatization, 3) unknowable information, and 4) impenetrability, which all go to elements of legitimacy. Possible improvements to Joint Implementation include adoption of the Gold Standard, enhanced public participation and administration, and limits on the use of Joint Implementation (supplementarity). None of these improvements can address the core problems related to additionality. Improvements that would meaningfully address the problems would probably also leave the mechanism of very limited use.