Abstract: A variable geometry turbine has a gas inlet chamber and a gas flow inlet passage downstream of the inlet chamber and upstream of said turbine wheel. The inlet passage is defined between a moveable first wall and a second wall and an actuator is operable to control displacement of the first wall in order to vary the size of the gas flow inlet passage. A piston member defining the first wall or being connected to it is received in a piston chamber and serves to move the first wall as a result of displacement of the piston member in the piston chamber. A bypass passage defined in the housing and extends between the piston chamber and a location in the turbine upstream of the inlet passage for delivering gas to the piston chamber. The bypass passage is disposed such that gas introduced through the bypass passage pressurises the chamber and applies a force to displace the piston member in a direction that causes the first wall to move towards the second wall.