In this paper, we discuss the greenhouse gas emission reduction potential from international shipping. Drawing from the International Maritime Organization’s most recent assessment of maritime greenhouse gas emissions and other sources, we investigate the current level of emissions from international maritime activity and look at factors influencing future emission levels such as projected activity levels, GHG-reducing technology options and the rate of their uptake, operational measures – foremost speed reduction – and fuel switching. We do not discuss the marginal abatement costs of maritime GHG-reduction measures – with the exception of speed reduction – due to insufficient evidence. Finally, we discuss factors that may influence international responses to maritime GHG reduction policies, though these are discussed more thoroughly in a companion paper (Kågeson, 2009). CO2 emissions from maritime transport are larger than has previously been estimated The IMO finds that international maritime activity accounted for 843 Mt of CO2 in 2007 or 45% more than previous emission estimates from marine bunkers. This finding, for illustrative purposes, places 2007 international shipping emissions between the 2005 national emissions of India and Germany. International shipping accounts for approximately 2.7% of world CO2 emissions from fossil fuel combustion with all shipping activity (fishing, domestic and international) representing approximately 3.3% of total CO2 from fuel combustion. Despite projected efficiency improvements, the IMO projects that CO2 emissions from international maritime activity will grow through 2050 though this growth may significantly slowed through uptake of fuel efficient technologies and operating procedures.