Hydrogen is a highly combustible, energetic gas that produces no GHGs when oxidized (combusted), widely used as a chemical industry and refinery feedstock. It can be used for direct process heating at all widely used temperatures, is a potential end-use fuel in internal combustion engines and turbines and is transformable into electricity using fuel cells for vehicles or stationary use. It is widely suggested as an alternative or complementary pathway to electrification for economy-wide decarbonization. How the hydrogen is made, however, is critical to its GHG impact. Current and future sourcing options can be divided into grey (fossil fuel-based), blue (fossil fuel-based production with carbon capture, utilisation and storage) and green (renewables-based) hydrogen. Green hydrogen is produced through renewable-powered electrolysis. Electrolysis has been commercialized since the 1880s using large and heavy alkaline electrolysers. However, there are two new ways to make green hydrogen that are entering the market: proton exchange membrane (PEM) electrolyser, which is lighter, smaller and more modular, and therefore better suited to vehicles; and solid oxide fuel cells (SOFC), which are larger, heavier and operate at higher temperatures, but are potentially more efficient and can operate in a dual directional mode. The key attribute of green hydrogen is that it allows a means for transforming variable intermittent wind and solar PV electricity into a highly useful, storable energy carrier that can also be transformed back to electricity as needed. It has the potential to add time and space option value to instantaneously generated wind and solar electricity for on-demand electricity, process heat, or chemical feedstocks (e.g. for making fertilizer, upgrading biogasification products, reducing iron ore to make steel). The potential contribution of green hydrogen could be very large but is highly contingent on the availability of relatively inexpensive renewable electricity.