An aircraft fuel tank ventilation system comprising a dehumidifying device (17) disposed in flow communication between a vent (15) open to the atmosphere and a fuel tank (4,5). Also, a method of dehumidifying air introduced into an aircraft fuel tank via a ventilation system, the method comprising directing atmospheric air through a dehumidifying device disposed in flow communication between a vent of the ventilation system and the fuel tank, which removes water vapour from the air flowing from the vent towards the fuel tank. Also, a method of regenerating a desiccant medium (22) used to dehumidify air introduced into an aircraft fuel tank via a ventilation system, the method comprising directing air through the desiccant medium so as to dry the desiccant medium. By effectively removing water from air entering the aircraft fuel tank, the content of water in the fuel tank can be kept low, which improves fuel system component reliability, reduces maintenance activities, and reduces microbiological contamination.