Flexible electricity systems allow a higher penetration of variable renewable energy, and flexibility can be achieved through pumped hydropower storage (PHS). This assessment of European PHS potential focuses on linking two existing reservoirs to form a PHS system, the reservoirs must have adequate difference in elevation (head) and be close enough so that they can be reasonably linked. The results show that the theoretical potential energy storage is significant as it reaches 54 TWh when a maximum distance of 20 km between the existing reservoirs is considered. When constraints are applied, e.g. discounting populated areas, protected natural areas or transport infrastructure, the so-called maximum realisable potential is halved to 29 TWh. Comparing with the existing PHS storage capacity reported for 14 countries suggests that the theoretical potential is 3.5 times the existing capacity, whereas the realisable potential is still twice the existing capacity. (C) 2014 Elsevier Ltd. All rights reserved.