This report documents the environmental performance of existing PHES and CAES, identifying factors affecting their performance. The work is based on an extensive literature study and on six case studies that have been chosen from five of the six partner countries; Austria, Germany, Greece, Ireland and Spain. No case could be chosen from the sixth partner country, Denmark, as it does not have any PHES or CAES. Efforts have been made to include different types of PHES plants in order to examine the full scope of environmental performance issues affecting PHES, as different schemes may have different associated environmental issues. This also ensures that the information is relevant to the greatest number of cases. Therefore one CAES, one closed-loop PHES, three semi-closed loop PHES and one open-system PHES have been selected for consideration. While the environmental impact of energy storage facilities associated with their development, design, construction, operation and decommissioning may