Pumped storage hydropower has grown rapidly over the last fifty years, first to store energy produced by thermal and nuclear sta-tions during off-peak hours when demand is low, and since the turn of the century to deal with the intermittency of wind and solar power generation. By 2023 the global installed capacity of pumped storage projects had reached 179 GW, 28.4% of which was in China, 15.3% in Japan and 12.4% in the United States. The International Hydropower Association (IHA) estimates that pumped storage accounts for over 94% of the world’s long duration energy storage capacity. Despite progress in the development of battery technolo-gy for large-scale energy storage and research in alternatives to lithium-ion batteries, such as sodium-ion batteries, flow batteries, and solid-state batteries, it is unlikely that any of these technologies will be cost-competitive with pump storage hydro in the near future.