A data centre (100) includes at least one rack room (in for example module 140) having a floor and a plurality of rack storage areas on the floor, each rack storage area being arranged to accommodate a plurality of racks (143) in which a plurality of rack-mountable electronic components may be housed, one or more controllable air circulation systems (in for example module 122), one or more cold aisles (144) in the rack room, each cold aisle being adjacent to a rack storage area, and one or more hot aisles (145) in the rack room, each hot aisle being adjacent to a rack storage area. There may be a large air duct, in the form of a personnel corridor (123), for transporting, under the control of the one or more air circulation systems, cooling air, above the floor, to the one or more cold aisles. The air supply corridor/duct (123) may have a height greater than 1.5m above the floor and a cross-sectional area of at least 2m 2 and a maximum dimension in the plane of the cross-section of less than 3m.