An energy storage system comprising: a storage volume for storing compressed gas; a multi-stage gas compressor configured to compress air that has been heated to a first temperature, the compression being such that the air leaving each compression stage reaches a second temperature; an expander configured to operate only during the discharge mode to extract work from the gas entering the expander at the second temperature and from which expander the exhaust gas emerges at a temperature significantly lower than the first temperature, wherein the gas comprises the air from the multi-stage gas compressor; one or more first thermal stores (1,2,3,4) configured to retain heat at temperatures in a range up to the first temperature and; one or more second thermal stores (1,2,3,4) configured to retain heat at temperatures in a range between the first temperature and second temperature; and a thermal capture for capturing heat from an external source for storing in the one or more first thermal stores at temperatures in a range up to the first temperature; wherein the system is operable to store energy in a charging mode, the charging mode comprising two or more different charging processes that are independently operable; and wherein the two or more charging processes comprise: a first charging process in which the air is compressed in the multi-stage compressor to fill the storage volume with pressurized gas and the second thermal store is simultaneously filled with heat; and a second charging process in which the first thermal store is filled with heat; and wherein a total mass of gas expanded in the expander in a discharging mode is greater than a total mass of gas compressed in the multi-stage gas compressor in the first charging process. a storage volume for storing compressed gas; a multi-stage gas compressor configured to compress air that has been heated to a first temperature, the compression being such that the air leaving each compression stage reaches a second temperature; an expander configured to operate only during the discharge mode to extract work from the gas entering the expander at the second temperature and from which expander the exhaust gas emerges at a temperature significantly lower than the first temperature, wherein the gas comprises the air from the multi-stage gas compressor; one or more first thermal stores (1,2,3,4) configured to retain heat at temperatures in a range up to the first temperature and; one or more second thermal stores (1,2,3,4) configured to retain heat at temperatures in a range between the first temperature and second temperature; and a thermal capture for capturing heat from an external source for storing in the one or more first thermal stores at temperatures in a range up to the first temperature; wherein the system is operable to store energy in a charging mode, the charging mode comprising two or more different charging processes that are independently operable; and wherein the two or more charging processes comprise: a first charging process in which the air is compressed in the multi-stage compressor to fill the storage volume with pressurized gas and the second thermal store is simultaneously filled with heat; and a second charging process in which the first thermal store is filled with heat; and wherein a total mass of gas expanded in the expander in a discharging mode is greater than a total mass of gas compressed in the multi-stage gas compressor in the first charging process. a first charging process in which the air is compressed in the multi-stage compressor to fill the storage volume with pressurized gas and the second thermal store is simultaneously filled with heat; and a second charging process in which the first thermal store is filled with heat; and wherein a total mass of gas expanded in the expander in a discharging mode is greater than a total mass of gas compressed in the multi-stage gas compressor in the first charging process.