A system is disclosed for controlling the reactant and coolant streams to and from a fuel cell, of the type having a first reactant inlet, a first reactant outlet, a second reactant inlet, a second reactant outlet, a coolant inlet and a coolant outlet. The system comprises: (a) a first reactant humidification sub-system for supplying a first reactant inlet stream to the first reactant inlet of the fuel cell and receiving a first reactant exhaust stream from the first reactant outlet of the fuel cell, where the first reactant humidification sub-system consists of an enthalpy means for collecting moisture from the first reactant exhaust stream and transferring a portion of the collected moisture to the first reactant inlet stream; (b) a second reactant humidity retention sub-system consisting of a re-circulation loop for collecting excess second treatment from the second reactant outlet, a source of second reactant, a means for mixing the second reactant from the source with that collected from the second reactant outlet of the fuel cell, and a means for circulating the second reactant in the re-circulation loop and for introducing second reactant into the second reactant inlet of the fuel cell; and (c) a coolant sub-system consisting of a first coolant loop joined to the coolant inlet and the coolant outlet, a second coolant loop and a heat-exchanger for transferring heat between the first and second coolant loops, where the first coolant loop is segregated from the second coolant loop. A method for controlling the reactant and coolant streams to and from a fuel cell as above. A system for conditioning a stream of reactant supplied to a fuel cell. A method for conditioning air supplied as an oxidant to a fuel cell's cathode. A method for retaining moisture in a flow of hydrogen to an anode of a fuel cell. A coolant system for cooling a fuel cell.    USE -   Used for managing and controlling the various flow streams related to the operation of a fuel cell, particularly for automotive applications.    ADVANTAGE -   Provides greatly reduced weight and complexity compared to known fuel-cell gas-management systems. A stainless plate heat-exchanger is used for de-ionised to ordinary water/ethylene glycol heat-transfer. This enables a cheaper and lighter conventional car radiator to be used.