A two stage turbocharger assembly for supplying charge air to an internal combustion engine, including: first and second stage turbochargers (10, 20), and an aftercooler; the first stage turbocharger having a first compressor wheel driven in rotation by a first turbine wheel; the second stage turbocharger having a second compressor wheel driven in rotation by a second turbine wheel; the first and second turbine wheels (12, 22) being arranged in series in an exhaust flowpath to be driven in rotation by exhaust gas flowing through the exhaust flowpath; the second compressor wheel being arranged downstream of the first compressor wheel in a charge air flowpath to compress charge air flowing through the charge air flowpath; the aftercooler being arranged to cool the charge air flowing through the charge air flowpath downstream of the second compressor wheel; a wastegate openable to allow exhaust gas flowing through the exhaust flowpath to bypass the second turbine wheel, an actuator control system, and an actuator operable responsive to the actuator control system to open and close the wastegate; characterized in that the two stage turbocharger assembly further includes a cooling flowpath, the cooling flowpath being arranged to direct a flow of charge air from the charge air flowpath downstream of the aftercooler to the second compressor wheel to cool the second compressor wheel; and further characterized in that the actuator is operable by charge air from the cooling flowpath. first and second stage turbochargers (10, 20), and an aftercooler; the first stage turbocharger having a first compressor wheel driven in rotation by a first turbine wheel; the second stage turbocharger having a second compressor wheel driven in rotation by a second turbine wheel; the first and second turbine wheels (12, 22) being arranged in series in an exhaust flowpath to be driven in rotation by exhaust gas flowing through the exhaust flowpath; the second compressor wheel being arranged downstream of the first compressor wheel in a charge air flowpath to compress charge air flowing through the charge air flowpath; the aftercooler being arranged to cool the charge air flowing through the charge air flowpath downstream of the second compressor wheel; a wastegate openable to allow exhaust gas flowing through the exhaust flowpath to bypass the second turbine wheel, an actuator control system, and an actuator operable responsive to the actuator control system to the first stage turbocharger having a first compressor wheel driven in rotation by a first turbine wheel; the second stage turbocharger having a second compressor wheel driven in rotation by a second turbine wheel; the first and second turbine wheels (12, 22) being arranged in series in an exhaust flowpath to be driven in rotation by exhaust gas flowing through the exhaust flowpath; the second compressor wheel being arranged downstream of the first compressor wheel in a charge air flowpath to compress charge air flowing through the charge air flowpath; the aftercooler being arranged to cool the charge air flowing through the charge air flowpath downstream of the second compressor wheel; a wastegate openable to allow exhaust gas flowing through the exhaust flowpath to bypass the second turbine wheel, an actuator control system, and an actuator operable responsive to the actuator control system to open and close the wastegate; characterized in that the two stage turbocharger assembly further includes a cooling flowpath, the cooling flowpath being arranged to direct a flow of charge air from the charge air flowpath downstream of the aftercooler to the second compressor wheel to cool the second compressor wheel; and further characterized in that the actuator is operable by charge air from the cooling flowpath.