A control system for controlling the amount of water in at least one fluid stream that is passed through a fuel cell stack comprises a humidifier arrangement, a sensor and a controller. The humidifier arrangement is operable to humidify the fluid stream. The sensor is disposed downstream of the humidifier arrangement and configured to measure at least one characteristic of the fluid stream. The at least one characteristic of fluid stream is indicative of the amount of water in the fluid stream. The controller is configured to receive a signal from the sensor and to use the signal to determine a difference between an amount of water in the fluid stream and a target amount of water. The controller is operable to control the humidifier arrangement to control the amount of water in the fluid stream based on the difference.