The use of hydrogen as a substitute for carbon-containing fossil fuels, such as natural gas, would lead to reductions in carbon dioxide and methane emissions and therefore have a significant climate benefit. However, leakage of hydrogen into the atmosphere during its production, storage, distribution and use will affect atmospheric composition, reducing the climate benefit and impacting air quality. A 2018 literature review, funded by the Department for Business, Energy and Industrial Strategy (BEIS), identified a number of evidence gaps which prevent accurate assessment of hydrogen’s environmental impact. These need to be urgently addressed to understand the implications of hydrogen use and enable unintended consequences of the delivery of a hydrogen economy to be minimised. This research programme aims to fill these through the following topics: