A heat storage system (400) comprising a system gas inlet (460), a system gas outlet (470), and at least two thermal stores (401, 402) connected together in series therebetween, wherein each store comprises a chamber having a gas inlet (461,462), a gas outlet (471,472), and a gas-permeable thermal storage media 431 disposed therebetween, the system further comprising flow controllers (451, 452, 453, 454, 457) operatively connected to bypass passageways and so configured that, during operation, the flow path of a gas flowing through the system (400) for transfer of thermal energy to or from the storage media (431) can be selectively altered in respect of which stores (401, 402) in the series are used in response to the progress of the thermal transfer.