This report quantifies the extent to which energy efficiency measures can reduce greenhouse gas (GHG) emissions and fuel costs in global shipping. Drawing on a fleet-wide analysis across key vessel segments (bulk carriers, container ships, and tankers), it assesses the untapped potential of technical and operational efficiency measures through to 2050. Findings show that maximizing energy efficiency can cut global shipping’s GHG emissions by up to about 40% by 2030, exceeding current IMO interim targets, while simultaneously lowering the costs of the energy transition. Roughly half of these potential GHG savings by 2030 pay for themselves, offering savings of up to $220 billion annually in total costs as green fuel supply chains develop, and helping to build resiliency against fuel price volatility and rerouting shocks. The report highlights the role of short-term operational measures (such as forms of port call and speed optimization) and medium-term technical innovations (for example, wind-assisted propulsion) in achieving substantial efficiency gains. It identifies persistent economic, behavioral, and organizational barriers to uptake and illustrates them through deep dives on port call optimization and wind-assisted propulsion, showcasing innovative industry initiatives being applied to overcome these barriers. Finally, the report offers targeted recommendations for policymakers, industry, ports, and financiers to accelerate the adoption of energy efficiency solutions at scale.