The "Clean Development Mechanism" (CDM) contained in the December 1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change provides, for the first time, the capacity for industrialized countries to claim credits for greenhouse gas (GHG) emissions reductions or offsets undertaken in cooperation with host developing countries. However, the Protocol provides no guidance on how these cooperative activities for GHG reduction and sustainable development would be undertaken in practice, including the particularly important issue of the relationship of the private sector vis-à-vis government institutions in designing, financing, and securing approval for jointly implemented GHG abatement projects. The pilot program for "Activities Implemented Jointly" under the Framework Convention provides an opportunity to better understand the practical constraints and opportunities for successful private sector participation in the CDM. This paper highlights some of the lessons for establishing a successful CDM by examining a small number of cases from the United States Initiative on Joint Implementation (USIJI). We first review the objectives, proposal review and evaluation criteria of this program, and provide some overall information on project proposals by project type and stage of development. We then develop case studies of two energy-related USIJI projects from the earlier phase of the program. These cases illustrate several potential problems that can arise in establishing CDM transactions. Further investigation of more recent cases sheds some light on the extent to which these problems change over time. To be successful, the CDM must be based on a solid institutional footing, with clear incentives for all parties involved. The cases we examine illustrate how transactions can become entangled in the same kinds of problems that bedevil other transactions in developing and transitional economies. In both early cases, "transaction costs" were substantial. The latter projects indicated that while the nature of transactions costs changed over time, they still remained somewhat substantial. Project proponents regarded gaining USIJI acceptance as one of the principal impediments to JI project development. The cases also illustrate the need for clear and widely understood goals and procedures for investor country approval. In addition, the analysis underscores how attitudes of different project proponents regarding the value of GHG credits can affect their perspective on the transaction. Finally, the study underscores that financing remains the ultimate hurdle to project implementation, and that the expectation of a clear financial return on investment is a prerequisite to a successful project.