First and second pump stages provide a flow-path from an inlet to the outlet (30), the flow-path being arranged so that molecules entering the first inlet (26) pass to the outlet through the first (120) and second (122) pump stage, and so that molecules entering the second inlet (28) pass to the outlet through an inter-stage volume (121) and second pump stage (122); wherein the first (120) and second (122) pump stages each comprise a turbo-molecular sub-stage (120a, 122a) and a molecular drag sub-stage (120b, 122b).