Producing green hydrogen at a cost that is competitive with fossil fuel-derived hydrogen is one of the major challenges for transitioning to a hydrogen economy. There are promising technologies for converting solar energy and water to hydrogen, such as coupling of photovoltaic and electrolysis (PV+E), photoelectrochemical (PEC) and (particulate) photocatalytic (PC) water splitting (see Figure ). These technologies, however, cannot currently produce hydrogen at a cost that is competitive with hydrogen derived from steam reforming. Hence, the question arises what and how much technological progress is required to reach a point of competitiveness? Understanding these requirements can inform (materials) research efforts, focusing them on the areas that are most impactful for quickly improving green hydrogen production routes.