This paper identifies and analyses options for the design of the Article 6.4 mechanism in two key areas. These are the possible transition of eligible activities registered under the Kyoto Protocol’s Clean Development Mechanism (CDM) to the Article 6.4 mechanism; and the registration of new activities under the Article 6.4 mechanism. The paper outlines possible transition options and potential implications for four issues relating to host Party approval of activities and to the use, review and revision of baseline methodologies and accreditation standards. The paper also highlights the steps needed to register new or transitioned activities under the Article 6.4 mechanism, and how co-ordination between different actors can facilitate a transition. The paper concludes that there are options available to ensure that the Article 6.4 mechanism can be implemented within a few years of a formal agreement on the rules, modalities and procedures for Article 6, and can build on the significant experience gained with the CDM. The paper highlights different ways that this CDM experience can be built on, and outlines the varying administrative and environmental implications of doing so.