A variable geometry turbine comprising: a housing (2, 4); a turbine wheel supported in the housing for rotation about an Z axis, a movable wall member comprising a generally annular wall and radially inner and outer flanges (21, 22) extending axially from the generally annular wall; a cavity provided in the housing for receipt of the radially inner and outer flanges of the movable wall member, the movable wall member being axially movable relative to the housing to vary the extent to which the radially inner and outer flanges of the movable wall member are received in the cavity, the cavity being defined by radially inner and outer curved side surfaces (26, 27) and a base surface Z extending between the radially inner and outer curved side surfaces; an inlet passageway extending radially inwards towards the turbine wheel and defined between a face of the generally annular wall of the movable wall member and an opposing wall of the housing, such that said axial movement of the movable wall member relative to the housing varies the axial width of the inlet passageway; and a plurality of axially extending apertures (32, 33) provided through the generally annular wall of the movable wall member, such that the inlet and the cavity are in fluid communication via the plurality of apertures; wherein a profile shape of the base surface generally matches a profile shape of an interior surface of the movable wall member; and wherein the base surface of the cavity comprises at least one arcuate radially central portion which is shaped so as to be received in an interior of the movable wall member when it is disposed in a fully open position. a housing (2, 4); a turbine wheel supported in the housing for rotation about an Z axis, a movable wall member comprising a generally annular wall and radially inner and outer flanges (21, 22) extending axially from the generally annular wall; a cavity provided in the housing for receipt of the radially inner and outer flanges of the movable wall member, the movable wall member being axially movable relative to the housing to vary the extent to which the radially inner and outer flanges of the movable wall member are received in the cavity, the cavity being defined by radially inner and outer curved side surfaces (26, 27) and a base surface Z extending between the radially inner and outer curved side surfaces; an inlet passageway extending radially inwards towards the turbine wheel and defined between a face of the generally annular wall of the movable wall member and an opposing wall of the housing, such that said axial movement of the movable wall member relative to the housing varies the axial width of the inlet passageway; and a plurality of axially extending apertures (32, 33) provided through the generally annular wall of the movable wall member, such that the inlet and the cavity are in fluid communication via the plurality of apertures; wherein a profile shape of the base surface generally matches a profile shape of an interior surface of the movable wall member; and wherein the base surface of the cavity comprises at least one arcuate radially central portion which is shaped so as to be received in an interior of the movable wall member when it is disposed in a fully open position.