A turbine bypass valve comprising: a casing defining first and second casing ports (42, 44); and a valve cartridge mounted to the casing; wherein the valve cartridge comprises: first and second valve ports (48, 50); and a valve member, the valve member being movable between a first position in which there is a flow path between the first and second valve ports, and a second position in which the valve member substantially blocks said flow path between the first and second valve ports; wherein the valve cartridge is mounted to the casing such that the first valve port is aligned with the first casing port, and the second valve port is aligned with the second casing port; charcterised in that the valve cartridge comprises an anti-rotation feature configured to prevent relative rotation between the casing and the valve cartridge when the valve cartridge is mounted to the casing; wherein the casing comprises a second anti-rotation feature configured to engage the anti-rotation feature when the valve cartridge is mounted to the casing; and wherein the anti-rotation feature comprises a protrusion which projects outwardly from the valve cartridge and the second anti-rotation feature comprises a recess configured to receive said protrusion; or wherein the second anti-rotation feature comprises a protrusion and the anti-rotation feature comprises a recess configured to receive said protrusion. a casing defining first and second casing ports (42, 44); and a valve cartridge mounted to the casing; wherein the valve cartridge comprises: first and second valve ports (48, 50); and a valve member, the valve member being movable between a first position in which there is a flow path between the first and second valve ports, and a second position in which the valve member substantially blocks said flow path between the first and second valve ports; wherein the valve cartridge is mounted to the casing such that the first valve port is aligned with the first casing port, and the second valve port is aligned with the second casing port; charcterised in that the valve cartridge comprises an anti-rotation feature configured to prevent relative rotation between the casing and the valve cartridge when the valve cartridge is mounted to the casing; wherein the casing comprises a second anti-rotation feature configured to engage the anti-rotation feature when the valve cartridge is mounted to the casing; and wherein the anti-rotation feature comprises a protrusion which projects outwardly from the valve cartridge and the second anti-rotation feature comprises a recess configured to receive said protrusion; or wherein the second anti-rotation feature comprises a protrusion and the anti-rotation feature comprises a recess configured to receive said protrusion. first and second valve ports (48, 50); and a valve member, the valve member being movable between a first position in which there is a flow path between the first and second valve ports, and a second position in which the valve member substantially blocks said flow path between the first and second valve ports; wherein the valve cartridge is mounted to the casing such that the first valve port is aligned with the first casing port, and the second valve port is aligned with the second casing port; charcterised in that the valve cartridge comprises an anti-rotation feature configured to prevent relative rotation between the casing and the valve cartridge when the valve cartridge is mounted to the casing; wherein the casing comprises a second anti-rotation feature configured to engage the anti-rotation feature when the valve cartridge is mounted to the casing; and wherein the anti-rotation feature comprises a protrusion which projects outwardly from the valve cartridge and the second anti-rotation feature comprises a recess configured to receive said protrusion; or wherein the second anti-rotation feature comprises a protrusion and the anti-rotation feature comprises a recess configured to receive said protrusion. wherein the anti-rotation feature comprises a protrusion which projects outwardly from the valve cartridge and the second anti-rotation feature comprises a recess configured to receive said protrusion; or wherein the second anti-rotation feature comprises a protrusion and the anti-rotation feature comprises a recess configured to receive said protrusion.