The prospects offered by hydrogen as part of the energy transition and the decarbonization of the energy system are major topical issues. Although sources of natural hydrogen have been identified in various parts of the world, it is not possible to estimate at this time the potential of these sources, nor to assess their exploitation capacities without further exploration. Thus, hydrogen is not a primary energy source but should only be considered as an energy carrier. Most of this hydrogen, produced today from fossil resources mainly for industrial usage (including oil refining and ammonia synthesis), will have to be obtained tomorrow from decarbonized processes and used more widely for other industrial applications (notably to reduce the carbon footprint of steel and cement production) and for heavy mobility. Given that hydrogen production must be guided primarily by the need to reduce greenhouse gas emissions, this report aims to define what is meant by "decarbonized" hydrogen, which must take precedence over all carbon-based hydrogen. The aim of this report is to clarify how hydrogen can be produced with minimal emissions of greenhouse gases, consider the significant needs it will generate in terms of electrical energy production1, on this basis identify the most appropriate uses for it in the future and derive estimates of a reasonable level of hydrogen production and consumption. The production of hydrogen by water electrolysis, which appears to be a key element in terms of carbon dioxide emissions (CO2), is really decarbonized if the electricity employed for its production is low carbon (nuclear or renewable), which is far from being the case in Europe or at a worldwide level. For the time being, the European electricity mix is largely carbon-based, and its use to power electrolyzers would lead to CO2 emissions twice as high as those of the conventional methane synthesis process. With its remarkably low carbon electricity mix, France has a major asset in playing a pioneering role in the deployment of low carbon hydrogen, provided that the new electricity production capacities required are rapidly available and themselves low carbon. The present analysis underlines the importance of the industrial competitiveness challenge of developing electrolyzers with the highest possible performance, in the service of national energy sovereignty. Efforts in this field deserve to be supported by scientific and technological research into the energy efficiency of electrolyzers and fuel cells, issues relating to reducing the environmental footprint of these components, improving their stability and lifespan, and, more generally, all the elements in the value chain (tanks, new materials, materials and molecules for storing and transporting hydrogen, etc.). The report also highlights the need to guide choices and developments through life-cycle analyses carried out across the entire value chain. The safety issues in using hydrogen are of major importance. New scientific and technological knowledge is essential if one wishes to define safe hydrogen applications. For applications envisaged outside industrial areas, one has to ensure that protocols and regulations remain compatible with their dissemination. Analysis of the future uses of carbon-free hydrogen indicates that, applications should initially be mainly in: (i) the industrial field, essentially to defossilize the industrial processes that emit the largest amounts of greenhouse gases (notably steel and cement production) and to replace grey hydrogen in current industrial uses (synthesis of ammonia and methanol); (ii) the field of heavy transport (sea or air), notably by enabling the synthesis of alternative fuels to replace current fossil fuels.