A turbocharger comprises a turbine wheel (5) mounted within a housing assembly (1, 3) for rotation about a turbine axis, a gas flow inlet passage (8) upstream of the turbine wheel (5), and a gas flow control mechanism (10, 16, 32) located within the housing assembly (1, 3) upstream of the turbine wheel (5) and operable to control gas flow through said gas flow inlet passage (9). An actuator assembly is mounted to the housing assembly and comprises: A lead screw including a lead screw shaft (37) defining a lead screw axis, and a lead screw nut (36) threadedly engaged on a threaded portion of the lead screw shaft (37). The lead screw (36, 37) being arranged so that rotation of one of said shaft and nut relative to the housing assembly produces linear movement in the other of said shaft and nut in the direction of the lead screw axis: The lead screw (36, 37) engages the gas flow control mechanism (10, 16, 32) at a location within a housing cavity defined within said housing assembly (1, 3).