A compressor for compressing a gas, the compressor comprising: a housing defining an inlet and an outlet; an impeller wheel including a plurality of vanes rotatably mounted within the housing; the housing having an inner wall defining a surface located in close proximity to radially outer edges of impeller vanes which sweep across said surface as the impeller wheel rotates about its axis; wherein the inlet comprises: an outer tubular wall extending away from the impeller wheel in an upstream direction and forming a gas intake portion of the inlet; an inner tubular wall of inner diameter D extending away from the impeller wheal in an upstream direction within the outer tubular wall and defining an inducer portion of the inlet; an annular gas flow passage defined between the inner and outer tubular walls and having an upstream end and a downstream end separated by a length L1 measured along its axis, the upstream end of the annular passage communicating with the intake or inducer portions of the inlet through at least one upstream aperture; at least one downstream aperture communicating between a downstream portion of the annular flow passage and said surface of the housing swept by the impeller vanes; the Inner tubular wall extending upstream of said at least one downstream aperture by a length L2 measured along its axis; wherein L1/D > 0.65 and/or L2/D > 0.6; characterised In that the diameter D is constant along substantially the entire length L2 of the inner tubular wall. a housing defining an inlet and an outlet; an impeller wheel including a plurality of vanes rotatably mounted within the housing; the housing having an inner wall defining a surface located in close proximity to radially outer edges of impeller vanes which sweep across said surface as the impeller wheel rotates about its axis; wherein the inlet comprises: an outer tubular wall extending away from the impeller wheel in an upstream direction and forming a gas intake portion of the inlet; an inner tubular wall of inner diameter D extending away from the impeller wheal in an upstream direction within the outer tubular wall and defining an inducer portion of the inlet; an annular gas flow passage defined between the inner and outer tubular walls and having an upstream end and a downstream end separated by a length L1 measured along its axis, the upstream end of the annular passage communicating with the intake or inducer portions of the inlet through at least one upstream aperture; at least one downstream aperture communicating between a downstream portion of the annular flow passage and said surface of the housing swept by the impeller vanes; the Inner tubular wall extending upstream of said at least one downstream aperture by a length L2 measured along its axis; wherein L1/D > 0.65 and/or L2/D > 0.6; characterised In that the diameter D is constant along substantially the entire length L2 of the inner tubular wall. an outer tubular wall extending away from the impeller wheel in an upstream direction and forming a gas intake portion of the inlet; an inner tubular wall of inner diameter D extending away from the impeller wheal in an upstream direction within the outer tubular wall and defining an inducer portion of the inlet; an annular gas flow passage defined between the inner and outer tubular walls and having an upstream end and a downstream end separated by a length L1 measured along its axis, the upstream end of the annular passage communicating with the intake or inducer portions of the inlet through at least one upstream aperture; at least one downstream aperture communicating between a downstream portion of the annular flow passage and said surface of the housing swept by the impeller vanes; the Inner tubular wall extending upstream of said at least one downstream aperture by a length L2 measured along its axis; wherein L1/D > 0.65 and/or L2/D > 0.6; characterised In that the diameter D is constant along substantially the entire length L2 of the inner tubular wall.