An exhaust gas-driven turbocharger, comprising: a shaft having a first end and a second end; a one-stage turbine wheel mounted on the shaft proximate the first end; a turbine housing surrounding the turbine wheel and defining ducting for directing exhaust gas into and out of the turbine wheel; a first-stage compressor wheel mounted on the shaft proximate the second end, the first-stage compressor wheel comprising first and second impellers disposed back-to-back; a second-stage compressor wheel mounted on the shaft at a location intermediate the first-stage compressor wheel and the turbine wheel; and a compressor housing assembly surrounding the first- and second-stage compressor wheels and defining: an inlet ducting configured to direct two separate portions of air in generally opposite axial directions into respective inlets of the first and second impellers of the first-stage compressor wheel; an interstage ducting configured to direct pressurized air from an exit of each of the impellers of the first-stage compressor wheel into an inlet of the second-stage compressor wheel; and a discharge ducting configured to collect pressurized air discharged from the second-stage compressor wheel for supply to an intake of an internal combustion engine; wherein the first- and second-stage compressor wheels and the turbine wheel are all fixedly mounted on the shaft and therefore rotate at the same angular rate of rotation, wherein: the inlet ducting comprises a common duct configured to conduct air for both the first and second impellers of the first-stage compressor wheel, a first duct configured to conduct one portion of the air from the common duct into the inlet of the first impeller, and a plurality of separate second ducts configured to conduct a remaining portion of the air from the common duct into the inlet of the second impeller; and the second ducts are circumferentially spaced apart from one another about a rotational axis defined by the shaft. a shaft having a first end and a second end; a one-stage turbine wheel mounted on the shaft proximate the first end; a turbine housing surrounding the turbine wheel and defining ducting for directing exhaust gas into and out of the turbine wheel; a first-stage compressor wheel mounted on the shaft proximate the second end, the first-stage compressor wheel comprising first and second impellers disposed back-to-back; a second-stage compressor wheel mounted on the shaft at a location intermediate the first-stage compressor wheel and the turbine wheel; and a compressor housing assembly surrounding the first- and second-stage compressor wheels and defining: an inlet ducting configured to direct two separate portions of air in generally opposite axial directions into respective inlets of the first and second impellers of the first-stage compressor wheel; an interstage ducting configured to direct pressurized air from an exit of each of the impellers of the first-stage compressor wheel into an inlet of the second-stage compressor wheel; and a discharge ducting configured to collect pressurized air discharged from the second-stage compressor wheel for supply to an intake of an internal combustion engine; an inlet ducting configured to direct two separate portions of air in generally opposite axial directions into respective inlets of the first and second impellers of the first-stage compressor wheel; an interstage ducting configured to direct pressurized air from an exit of each of the impellers of the first-stage compressor wheel into an inlet of the second-stage compressor wheel; and a discharge ducting configured to collect pressurized air discharged from the second-stage compressor wheel for supply to an intake of an internal combustion engine; wherein the first- and second-stage compressor wheels and the turbine wheel are all fixedly mounted on the shaft and therefore rotate at the same angular rate of rotation, wherein: the inlet ducting comprises a common duct configured to conduct air for both the first and second impellers of the first-stage compressor wheel, a first duct configured to conduct one portion of the air from the common duct into the inlet of the first impeller, and a plurality of separate second ducts configured to conduct a remaining portion of the air from the common duct into the inlet of the second impeller; and the second ducts are circumferentially spaced apart from one another about a rotational axis defined by the shaft. the inlet ducting comprises a common duct configured to conduct air for both the first and second impellers of the first-stage compressor wheel, a first duct configured to conduct one portion of the air from the common duct into the inlet of the first impeller, and a plurality of separate second ducts configured to conduct a remaining portion of the air from the common duct into the inlet of the second impeller; and the second ducts are circumferentially spaced apart from one another about a rotational axis defined by the shaft.