Abstract: The production of hydrogen through water electrolysis (WE) from renewable electricity is set to revolutionise the energy sector that is at present heavily dependent on fossil fuels. However, there is still a pressing need to develop advanced electrocatalysts able to show high activity and withstand industrially-relevant operating conditions for a prolonged period of time. In this regard, high entropy materials (HEMs), including high entropy alloys and high entropy oxides, comprising five or more homogeneously distributed metal components, have emerged as a new class of electrocatalysts owing to their unique properties such as low atomic diffusion, structural stability, a wide variety of adsorption energies and multi-component synergy, making them promising catalysts for challenging electrochemical reactions, including those involved in WE. This review begins with a brief overview about WE technologies and a short introduction to HEMs including their synthesis and general physicochemical properties, followed by a nearly exhaustive summary of HEMs catalysts reported so far for the hydrogen evolution reaction, the oxygen evolution reaction and the overall water splitting in both alkaline and acidic conditions. The review concludes with a brief summary and an outlook about the future development of HEM-based catalysts and further research to be done to understand the catalytic mechanism and eventually deploy HEMs in practical water electrolysers.