A turbocharger comprising a centrifugal impeller, a turbine, a shaft and a casing system which houses the centrifugal impeller connected to the turbine by the shaft; the casing system including: an insert casing which forms a duct for feeding air to and through the impeller, a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing, and a main casing which forms a housing for the shaft and for the shaft-side end of the impeller; the impeller including: a hub which has a radially outer annular rim, and has a front face and a rear face which converge towards the rim from respectively the inlet side and the shaft side of the hub, and a plurality of circumferentially arranged vanes on the front face of the hub; a seal formation formed on the rear face at the rim; and the casing system further including: a seal plate having a corresponding seal formation which sealingly interacts with the impeller seal formation, the seal plate extending inboard from the rim to an attachment flange which attaches to the main casing, and extending outboard from the rim to carry or form a rear wall of an annular passage directing compressed air from the impeller to the volute; characterised in that the thickness of the seal plate is narrowed in a substantially cylindrical waist region adjacent the rim, the thickness of the waist region as measured in the radial direction, being less than 2.5 times the minimum thickness, as measured in the axial direction, of the rim, whereby cracking of the seal plate around the waist region is promoted in the event of an impeller burst. an insert casing which forms a duct for feeding air to and through the impeller, a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing, and a main casing which forms a housing for the shaft and for the shaft-side end of the impeller; the impeller including: a hub which has a radially outer annular rim, and has a front face and a rear face which converge towards the rim from respectively the inlet side and the shaft side of the hub, and a plurality of circumferentially arranged vanes on the front face of the hub; a seal formation formed on the rear face at the rim; and the casing system further including: a seal plate having a corresponding seal formation which sealingly interacts with the impeller seal formation, the seal plate extending inboard from the rim to an attachment flange which attaches to the main casing, and extending outboard from the rim to carry or form a rear wall of an annular passage directing compressed air from the impeller to the volute; characterised in that the thickness of the seal plate is narrowed in a substantially cylindrical waist region adjacent the rim, the thickness of the waist region as measured in the radial direction, being less than 2.5 times the minimum thickness, as measured in the axial direction, of the rim, whereby cracking of the seal plate around the waist region is promoted in the event of an impeller burst. a hub which has a radially outer annular rim, and has a front face and a rear face which converge towards the rim from respectively the inlet side and the shaft side of the hub, and a plurality of circumferentially arranged vanes on the front face of the hub; a seal formation formed on the rear face at the rim; and the casing system further including: a seal plate having a corresponding seal formation which sealingly interacts with the impeller seal formation, the seal plate extending inboard from the rim to an attachment flange which attaches to the main casing, and extending outboard from the rim to carry or form a rear wall of an annular passage directing compressed air from the impeller to the volute; characterised in that the thickness of the seal plate is narrowed in a substantially cylindrical waist region adjacent the rim, the thickness of the waist region as measured in the radial direction, being less than 2.5 times the minimum thickness, as measured in the axial direction, of the rim, whereby cracking of the seal plate around the waist region is promoted in the event of an impeller burst. a seal plate having a corresponding seal formation which sealingly interacts with the impeller seal formation, the seal plate extending inboard from the rim to an attachment flange which attaches to the main casing, and extending outboard from the rim to carry or form a rear wall of an annular passage directing compressed air from the impeller to the volute; characterised in that the thickness of the seal plate is narrowed in a substantially cylindrical waist region adjacent the rim, the thickness of the waist region as measured in the radial direction, being less than 2.5 times the minimum thickness, as measured in the axial direction, of the rim, whereby cracking of the seal plate around the waist region is promoted in the event of an impeller burst.