This presentation gives an overview how existing vessels can be retrofitted from traditional propulsion systems like Diesel engines, to zero-emission by using modular, heavy duty fuel cell systems. The necessary steps and required technologies will be explained based on the example project “HyEkoTank”, where a 18.600 DWT product tanker will be retrofitted by installing a 2.4 MW fuel cell system from TECO2030. Although international shipping is critical to global trade, maritime transport is a major source of air pollution, including greenhouse gas (GHG) emissions. Despite progress in recent years, the maritime sector still relies almost entirely on fossil fuels. In April 2018, the IMO adopted a non-binding agreement to reduce CO2 emissions by 40% until 2030 and by 70% until 2050. Suggested carbon reduction strategies include improving fuel quality and engine emission standards as well as using new technologies, such as fuel cells. The majority of new ships being build today are equipped with a conventional propulsion system (e.g. Diesel engine). As the typical lifetime of a ship is around 30 years, currently build vessels will still be sailing in 2050 and without modifications, not contributing to the fulfilment of the above IMO targets. Today there are > 102,000 existing vessels operating on the world’s seas and 1,500 – 3,000 new builds per year. It is therefore a great and ever-increasing need for technological innovation in zero-emission solutions developed for maritime transport, in particular for retrofitting existing ships. Switching from traditional petroleum-based marine fuels to zero-carbon alternatives such as hydrogen operated fuel cells can drastically shrink shipping’s climate impact. This session will give an overview how the maritime industry can be transformed by retrofitting existing vessels to zero-emission fuel cells.