The present invention concerns systems for storing energy and using the stored energy to generate electrical energy or drive a propeller (505). In particular, the present invention provides a method of storing energy comprising: providing a gaseous input, producing a cryogen from the gaseous input; storing the cryogen; expanding the cryogen; using the expanded cryogen to drive a turbine (320) and recovering cold energy from the expansion of the cryogen. The present invention also provides a cryogenic energy storage system comprising: a source of cryogen; a cryogen storage facility (370); means for expanding the cryogen; a turbine (320) capable of being driven by the expanding cryogen; and means (340, 350) for recovering cold energy released during expansion of the cryogen.