International shipping is responsible for 3% of global GHG emissions. Ships departing U.S. ports are responsible for 4.8% of international shipping emissions. U.S. domestic shipping (ships traveling from one U.S. port to another U.S. port) is responsible for only 0.08% of U.S. domestic emissions. International shipping emissions are projected to increase between 50% and 250% by 2050. This means that, on a business-as-usual pathway, total international shipping emissions could reach 18% of global GHG emissions by 2050. Yet, the DDPP reports for the United States do not look closely at shipping. The four 2050 scenarios in the reports all show energy demand for domestic shipping reducing by two-fifths but without much detail on how these reductions are achieved. International shipping is not considered in the reports. The specialized United Nations body for international shipping, the International Maritime Organization (IMO), has estimated that design, operational, and alternative fuel measures could reduce international shipping emissions by 75% by 2050 and that innovative technologies could achieve even further reductions. The legal issues that arise when considering the decarbonization of shipping are mainly jurisdictional. There are three types of authority that countries can exercise over ships: flag (regulating ships that fly the country’s flag), coastal (regulating the ships that pass through a country’s coastal waters), and port (regulating ships that dock in that country’s ports). The chapter concludes that the latter, regulating the ships that dock in U.S. ports (referred to as port jurisdiction under international law), is the most appropriate for controlling GHG emissions.