Hydrogen is a high quality energy carrier that could be produced at global scale, via thermochemical processing of hydrocarbons, such as natural gas, coal or biomass, or water electrolysis using any source of electricity including renewables, such as wind or solar, or nuclear power. Hydrogen is receiving renewed attention driven by growing concerns about climate change, air quality and integration of variable renewable energy into the energy system. Recent energy/economic studies suggest that hydrogen and fuel cells could be important technologies for simultaneously addressing these challenges in a future renewable-intensive, low carbon energy system. In this paper, we review the technical and economic status of hydrogen and fuel cell technologies, progress toward commercialization, and the role of policy. We discuss timing, barriers, costs and benefits of a hydrogen transition, focusing on vehicle and energy storage applications. Finally, we suggest guidelines for future policies guiding a hydrogen transition.