Aviation is a substantial and growing driver of climate change, currently responsible for almost 5% of global warming. The objectives of the Paris Agreement cannot be achieved without action to rein in its emissions growth. This T&E briefing outlines how, at its triennial assembly, ICAO has an opportunity to adopt a global market-based measure which can be a starting point for greater global ambition. However, negotiations dominated by the need to protect industry and favour historic emitters is weakening the prospect of a credible deal.