This short study seeks to contribute to the elaboration of the rules, modalities and procedures of the Article 6.4 mechanism. The transition from the Kyoto Protocol to the Paris Agreement has potentially strong impacts on the continuation of existing mitigation activities established through the CDM. This creates the need for a careful and selective transition process of methodologies and activities from the CDM towards the Article 6.4 mechanism. The study identifies key open issues and challenges concerning the transition of CDM methodologies, activities and units that Article 6 negotiators need to address in designing the transition process.