This report provides an independent assessment of the methods and technologies with the potential to reduce emissions in the maritime sector, focusing particularly on solutions for improvements in energy efficiency. Energy efficiency should always be prioritised and is a critical pathway for reducing emissions in the near term while more advanced decarbonisation technologies continue to mature. The report consolidates peer reviewed and grey literature to assess the current state of the art and uptake of GHG abatement options for the maritime sector. Firstly, the report outlines the opportunities, options and the key barriers to maritime decarbonisation. Secondly, the report examines the IMO regulations which aim to incentivize the implementation of energy efficiency measures and technologies to improve energy efficiency of ships. Thirdly, the report explores energy efficiency technologies and practices in detail, introducing each technology, the energy saving potential of the technology, the maturity of the technology and the current deployment of the technology. This report collates extensive information to provide an overview of maritime decarbonisation including a review of IMO’s initiatives and an analysis of measures to improving energy efficiency in the maritime sector. The report includes over 200 references, with thousands of sources reviewed and processed. Overall, the CII, EEXI and EEDI are key drivers of change, aimed at reducing GHG emissions and pollution from vessels and ports. To support these goals, a wide range of technologies and operational measures are available which have the potential to make significant energy savings.