A turbocharger comprising: a compressor wheel mounted within a compressor housing; a turbine housing defining at least part of a generally annular chamber extending circumferentially about a longitudinal axis of the turbine housing, the turbine housing defining a bore extending along the longitudinal axis; a turbine wheel disposed in the turbine housing; a nozzle passage leading from the generally annular chamber radially inwardly into the turbine wheel; a vane assembly for guiding exhaust gas flowing toward the turbine wheel, wherein the vane assembly is a fixed vane assembly comprising a plurality of fixed vanes disposed in the nozzle passage; the turbine housing defining an annular bypass passage surrounding the bore and arranged to allow exhaust gas in the generally annular chamber to bypass the turbine wheel; and an annular bypass valve disposed in the bypass passage, the bypass valve comprising a fixed annular valve seat which comprises a separate flat ring-shaped member defining a plurality of circumferentially spaced first orifices extending axially therethrough, and a rotary annular valve member which comprises a flat ring-shaped member defining a plurality of circumferentially spaced second orifices extending axially therethrough, the valve member being disposed against the valve seat and being rotatable about the longitudinal axis for selectively varying a degree of alignment between respective first and second orifices (73', 75') defined through each of the valve seat and valve member, ranging from no alignment defining a closed condition of the bypass valve, to at least partial alignment defining an open condition of the bypass valve. a compressor wheel mounted within a compressor housing; a turbine housing defining at least part of a generally annular chamber extending circumferentially about a longitudinal axis of the turbine housing, the turbine housing defining a bore extending along the longitudinal axis; a turbine wheel disposed in the turbine housing; a nozzle passage leading from the generally annular chamber radially inwardly into the turbine wheel; a vane assembly for guiding exhaust gas flowing toward the turbine wheel, wherein the vane assembly is a fixed vane assembly comprising a plurality of fixed vanes disposed in the nozzle passage; the turbine housing defining an annular bypass passage surrounding the bore and arranged to allow exhaust gas in the generally annular chamber to bypass the turbine wheel; and an annular bypass valve disposed in the bypass passage, the bypass valve comprising a fixed annular valve seat which comprises a separate flat ring-shaped member defining a plurality of circumferentially spaced first orifices extending axially therethrough, and a rotary annular valve member which comprises a flat ring-shaped member defining a plurality of circumferentially spaced second orifices extending axially therethrough, the valve member being disposed against the valve seat and being rotatable about the longitudinal axis for selectively varying a degree of alignment between respective first and second orifices (73', 75') defined through each of the valve seat and valve member, ranging from no alignment defining a closed condition of the bypass valve, to at least partial alignment defining an open condition of the bypass valve.