Internal combustion engines running on ammonia are increasingly considered as important facilitator among research, industry and policy communities in reaching EU environmental goals. The overall aim of the Ammonia2–4 project is to demonstrate at full scale two types of dual fuel marine engines running on ammonia as main fuel: a 4-stroke engine and a 2-stroke medium-pressure ammonia fuel injection platform. Both engine innovations are expected to result in at least 80% less GHG emissions (including nitrous oxide emissions), NOx emissions below IMO Tier III regulations and a negligible ammonia slip below 10ppm (Euro 6 compliant). The project will go beyond purely technological developments and investigate a number of non-technical aspects crucial for a successful uptake of ammonia as marine fuel: health & safety, ammonia supply infrastructure, crew training, novel standardization pathways for measurement and reporting emissions from ammonia marine engines.