Summary: Hydrogen is a possible alternative to the internal combustion engine, alongside battery-powered vehicles, in the context of reducing greenhouse gas emissions associated with transport activities. The costs associated with hydrogen vehicles are currently high, even when considering the greenhouse gas emissions and other pollutants avoided by their use. Efforts to reduce these costs, which will determine the social and environmental desirability of hydrogen vehicles, face two challenges : the high cost of refueling, linked to the crucial problem of coordination between development of the vehicle fleet and refueling infrastructure; and high purchase prices, which may decrease when sufficient quantities generate experience effects. This policy brief argues that each of these two handicaps calls for a specific policy design : at a local level for coordination between actors, and at a European level to generate sufficient volumes. The example of hydrogen-powered urban buses offers a telling illustration of these issues.. Key points: The growing importance of the hydrogen sector has been encouraged by various initiatives in France. These initiatives are based on the idea of a regional ecosystem : around a city, a network of local communities, or even a department or a region. The example of hydrogen buses shows that the abatement costs induced by this technology are still too high. The problem lies both in the price of the vehicles and the supply of fuel. Reducing the costs associated with the supply of fuel requires the resolution of coordination problems linked to network effects, which calls for a response at the local level. Achieving vehicle purchase prices low enough to be competitive requires a European approach, which alone makes it possible to reach significant volumes.