As the world shifts towards cleaner mobility, retrofitting vehicles with zero-emission powertrains is gaining momentum. This mechanical operation involves replacing petrol/diesel engines with electric motors or hydrogen fuel cells. In Europe and Asia, public authorities are promoting retrofitting to reduce carbon emissions and comply with climate goals. The retrofit market is expanding rapidly, with startups offering tailored solutions for buses, trucks, and vans. In France, a national action plan supports electric retrofitting, aiming to significantly decrease greenhouse gas emissions. Additionally, the global market for automotive retrofit electric powertrains is expected to grow substantially by 2034, reflecting a rising demand for sustainable transportation solutions. The UNECE has established an informal working group to develop harmonized regulations for retrofit systems, focusing on all vehicle categories and powertrain types. This global collaboration aims to set minimum requirements, ensure performance standards, and support the deployment of retrofit systems worldwide, with a target completion date of 2027. Although hydrogen-powered vehicles are in early stages, their potential is promising, especially if green hydrogen prices decrease. The UNECE working group's activities will prioritize technological readiness and economic viability for different vehicle types, with the involvement of France, Spain, and other key players in the automotive industry.