A steam generator for generating a superheated fluid from a working fluid using a stream of heated gas, the steam generator comprising : a housing, which defines a gas flow path having an inlet at one, upstream end thereof into which a stream of heated gas is delivered and an outlet at the other, downstream end thereof; and a steam generation module which is disposed within the gas flow path of the housing, the steam generation module comprising a heat exchanger which receives a working fluid and is operative to raise the temperature of the working fluid to provide a saturated fluid, and a superheater which receives the saturated fluid from the heat exchanger and is operative to raise the temperature of the saturated fluid and provide a supersaturated fluid.