A turbocharger comprising: a two-stage series compressor comprising a compressor housing assembly, and a compressor wheel comprising a first impeller and a second impeller that are mounted on a shaft for rotation therewith, the first and second impellers being contained in the compressor housing assembly, the compressor housing assembly defining a first compressor flow path including a first air inlet that leads air into the first impeller, a first volute that collects compressed air that has passed through and been compressed by the first impeller in operation, a second compressor flow path including a second air inlet that leads air into the second impeller, and a second volute that collects compressed air that has passed through and been compressed by the second impeller in operation, and further comprising an interstage duct that connects the first volute to the second air inlet such that, in operation, air compressed by the first impeller is led by the interstage duct from the first volute into the second air inlet and is further compressed by the second impeller and delivered into the second volute; an exhaust gas-driven turbine comprising a turbine housing defining an axial bore therein and a turbine wheel affixed to the shaft and contained in the axial bore of the turbine housing, the turbine housing defining an exhaust gas inlet for receiving exhaust gas, a generally annular chamber arranged to receive exhaust gas from the exhaust gas inlet, a nozzle arranged to feed exhaust gas from the chamber generally radially inwardly to the turbine wheel, exhaust gas being discharged, in operation, from the turbine housing via the axial bore; a center housing disposed between the compressor housing assembly and the turbine housing, the center housing containing one or more bearings for the shaft; and an electric motor comprising a generally annular motor stator concentrically surrounding a motor rotor, the motor rotor being affixed to the shaft, wherein energizing of the electric motor rotatably drives the motor rotor so as to assist the turbine wheel in rotatably driving the two-stage series compressor, wherein: the compressor housing assembly comprises a first compressor housing containing the first impeller and defining the first volute, and a separately formed second compressor housing containing the second impeller and defining the second volute, characterized in that the electric motor is disposed between the first compressor housing and the second compressor housing. a two-stage series compressor comprising a compressor housing assembly, and a compressor wheel comprising a first impeller and a second impeller that are mounted on a shaft for rotation therewith, the first and second impellers being contained in the compressor housing assembly, the compressor housing assembly defining a first compressor flow path including a first air inlet that leads air into the first impeller, a first volute that collects compressed air that has passed through and been compressed by the first impeller in operation, a second compressor flow path including a second air inlet that leads air into the second impeller, and a second volute that collects compressed air that has passed through and been compressed by the second impeller in operation, and further comprising an interstage duct that connects the first volute to the second air inlet such that, in operation, air compressed by the first impeller is led by the interstage duct from the first volute into the second air inlet and is further compressed by the second impeller and delivered into the second volute; an exhaust gas-driven turbine comprising a turbine housing defining an axial bore therein and a turbine wheel affixed to the shaft and contained in the axial bore of the turbine housing, the turbine housing defining an exhaust gas inlet for receiving exhaust gas, a generally annular chamber arranged to receive exhaust gas from the exhaust gas inlet, a nozzle arranged to feed exhaust gas from the chamber generally radially inwardly to the turbine wheel, exhaust gas being discharged, in operation, from the turbine housing via the axial bore; a center housing disposed between the compressor housing assembly and the turbine housing, the center housing containing one or more bearings for the shaft; and an electric motor comprising a generally annular motor stator concentrically surrounding a motor rotor, the motor rotor being affixed to the shaft, wherein energizing of the electric motor rotatably drives the motor rotor so as to assist the turbine wheel in rotatably driving the two-stage series compressor, wherein: the compressor housing assembly comprises a first compressor housing containing the first impeller and defining the first volute, and a separately formed second compressor housing containing the second impeller and defining the second volute, characterized in that the electric motor is disposed between the first compressor housing and the second compressor housing. the compressor housing assembly comprises a first compressor housing containing the first impeller and defining the first volute, and a separately formed second compressor housing containing the second impeller and defining the second volute, characterized in that the electric motor is disposed between the first compressor housing and the second compressor housing.