This 2017 article in the Journal of Environmental Law examines private standards that aim to mitigate GHG emissions in shipping. These have emerged against a backdrop of regulatory inertia and the exclusion of international shipping from the Paris Climate Change Agreement. They are a product of complex governance arrangements and they have addressed areas of market failure that have held back fuel efficiency advances that are made possible by technological innovations. The authors conclude that these private standards hold considerable promise but suffer to different degrees from certain weaknesses, notably a lack of transparency, a low level of ambition and concerns about data reliability. This article examines these deficiencies together with the reasons for them, and assesses the role that law could play in addressing them. It argues that the conditions may be present for the mitigation of shipping’s GHG emissions to become a site of ‘hybrid’ governance, combining private standards and state/supra-state law in a productive way.