The Paris Agreement sets out a framework for all countries to decide on their nationally determined contributions (NDCs) and subsequently demonstrate they have implemented them. Article 6 of the Paris Agreement recognizes that countries may collaborate in implementing their respective NDCs, including through the use of internationally transferred mitigation outcomes (ITMOs). How such transfers of mitigation outcomes between countries will be accounted for will be crucial in ensuring that environmental integrity is preserved. Further guidance on ITMO accounting will be provided in the Paris “rulebook” to be considered at COP 24 in Katowice, Poland, in December 2018. This brief sets out several key inter-related aspects of the emerging accounting framework and examines a range of options and their implications for other aspects of the Paris Agreement.