A two stage turbocharger assembly for supplying charge air to an internal combustion engine, including: first and second stage turbochargers , 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 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 , 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 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 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 . <EOS>