Abstract: A compressor (16) comprises a rotor (40) having a plurality of circumferentially spaced radially outwardly extending rotor blades (42). A casing (44) surrounds the rotor (40) and rotor blades (42). The casing (44) has an inner surface (48) and a plurality of circumferentially spaced slots (50) are provided in the inner surface (48) of the casing (44). Each slot (50) has a length (L), a depth (D), a width (W), an angle (?) of inclination relative to the radial direction, an axial position (AP) relative to the rotor blades (42) and a circumferential position (CP) relative to an adjacent slot (50). The slots (50) are arranged such that at least one of the length (L), depth (D), width (W), angle (?) of inclination relative to the radial direction, axial position (AP) relative to the rotor blades (42) and circumferential position (CP) relative to an adjacent slot (50) varies circumferentially around the casing (44).