Vacuum system

A vacuum pumping system 1, and method of evacuating a chamber, comprises Siegbahn and regenerative pumps 2, 3 in series. The Siegbahn pump comprises a plurality of stages 6, with a first inlet 4 upstream of the Siegbahn pump, and a second inlet 5 downstream of at least a first Siegbahn stage, and such that fluid entering the second inlet completes at least a single pass of a Siegbahn stage. The Siegbahn and regenerative pumps may be co-axial and positioned on a common drive shaft 9. A pump, such as a turbo-molecular pump, may be provided upstream of the Siegbahn pump. The turbo-molecular pump may comprise at least two pump stacks in series, and a third inlet may be provided between the pump stacks.