A gas turbine engine comprising: a compressor section rotatable about an engine axis, the compressor section comprising a plurality of stages (44,52) that include a plurality of compressor blades and vanes; a combustor section downstream of the compressor section, the combustor section including at least one combustor component to be cooled; a turbine section downstream of the combustor section, the turbine section comprising a plurality of stages (46,54) that include a plurality of turbine blades and vanes; and at least one blade outer air seal positioned radially outward of the plurality of compressor or turbine blades, and wherein at least one of the compressor blades and vanes, the turbine blades and vanes, the combustor component, and the blade outer air seal includes a body extending between two circumferential sides (74,76), and between a leading edge and a trailing edge, and at least one cooling circuit to utilize fluid to cool the body, wherein the at least one cooling circuit comprises at least first and second cooling circuits each including an inlet, an outlet, and a passage (84,84a) extending between the inlet and the outlet that varies in cross-sectional area (90,100) between the inlet and outlet, wherein the passage of the first cooling circuit has a first variable cross sectional area and the passage of the second cooling circuit has a second variable cross sectional area that is different than the first variable cross-sectional area, and the inlets are in a radially outer external surface of the body and the outlets are in a radially inner external surface; characterised in that: the passage of the first cooling circuit extends in a generally axial direction between the inlet and the outlet and has a planar or curved tapered cross-sectional area which progressively decreases in size from the inlet to the outlet; and the passage of the second cooling circuit extends in a generally axial direction between the inlet and the outlet and has a planar or curved stepped cross-sectional area which progressively decreases in size from the inlet to the outlet. a compressor section rotatable about an engine axis, the compressor section comprising a plurality of stages (44,52) that include a plurality of compressor blades and vanes; a combustor section downstream of the compressor section, the combustor section including at least one combustor component to be cooled; a turbine section downstream of the combustor section, the turbine section comprising a plurality of stages (46,54) that include a plurality of turbine blades and vanes; and at least one blade outer air seal positioned radially outward of the plurality of compressor or turbine blades, and wherein at least one of the compressor blades and vanes, the turbine blades and vanes, the combustor component, and the blade outer air seal includes a body extending between two circumferential sides (74,76), and between a leading edge and a trailing edge, and at least one cooling circuit to utilize fluid to cool the body, wherein the at least one cooling circuit comprises at least first and second cooling circuits each including an inlet, an outlet, and a passage (84,84a) extending between the inlet and the outlet that varies in cross-sectional area (90,100) between the inlet and outlet, wherein the passage of the first cooling circuit has a first variable cross sectional area and the passage of the second cooling circuit has a second variable cross sectional area that is different than the first variable cross-sectional area, and the inlets are in a radially outer external surface of the body and the outlets are in a radially inner external surface; characterised in that: the passage of the first cooling circuit extends in a generally axial direction between the inlet and the outlet and has a planar or curved tapered cross-sectional area which progressively decreases in size from the inlet to the outlet; and the passage of the second cooling circuit extends in a generally axial direction between the inlet and the outlet and has a planar or curved stepped cross-sectional area which progressively decreases in size from the inlet to the outlet. the passage of the first cooling circuit extends in a generally axial direction between the inlet and the outlet and has a planar or curved tapered cross-sectional area which progressively decreases in size from the inlet to the outlet; and the passage of the second cooling circuit extends in a generally axial direction between the inlet and the outlet and has a planar or curved stepped cross-sectional area which progressively decreases in size from the inlet to the outlet.