Hydrogen can be produced from many different renewable and non-renewable feedstocks and technological pathways, with widely varying greenhouse gas emissions. For hydrogen to have a role in future low-carbon energy systems, it is necessary to demonstrate that it has sufficiently low carbon emissions. This paper explores how green hydrogen has been defined, reviews nascent green hydrogen characterisation initiatives, and highlights the main challenges that standards and guarantee of origin schemes must overcome to develop a market for green hydrogen. Most existing green hydrogen initiatives are in Europe. In anticipation of a future market for green hydrogen, international standards are starting to be discussed by national and international standardisation organisations and policy makers. A range of approaches have been taken to defining green hydrogen and guarantees of origin. These vary on whether green hydrogen must be produced from renewable energy, on the boundaries of the carbon accounting system, the emission thresholds at which hydrogen is considered green, and on which feedstocks and production technologies are included in the scheme. Decisions on these factors are often influenced by other national and international standards, and the legal framework in which the green hydrogen supply chain operates. Display omitted •There is not a universally accepted definition for green hydrogen.•Several green characterisation initiatives are under development.•Guarantees of origin demonstrate to consumers that they are using green hydrogen.•Harmonisation of guarantees of origin would facilitate international trade.•Policies must be aligned with those of other energy carriers to avoid discrimination.