A gas turbine engine comprising: a compressor section, a combustor, and a turbine section; and an associated fluid to be cooled, an associated fluid to be heated, and a transcritical vapor cycle for heating said fluid to be heated and for cooling said fluid to be cooled, said transcritical vapor cycle including a gas cooler, in which said fluid to be heated is heated by a refrigerant in said transcritical vapor cycle and an evaporator heat exchanger at which said fluid to be cooled is cooled by the refrigerant in said transcritical vapor cycle, a compressor upstream of said gas cooler for compressing the refrigerant to a pressure above a critical point for the refrigerant, and an expansion device for expanding the refrigerant downstream of said gas cooler, with said evaporator heat exchanger being downstream of said expansion device, and such that the refrigerant passing through said gas cooler to heat said fluid to be heated is generally above the critical point. wherein said fluid to be heated is fuel, characterised in that: said fuel to be heated is fuel passing to said combustor via a supply line, wherein the gas turbine engine further comprises a fuel system comprising said supply line, a return line, at least one return line heat exchanger, and a fuel tank, wherein said return line (206; 217) is downstream of said gas cooler for returning fuel to said fuel tank and said at least one return line heat exchanger (208, 214; 222, 224) is provided on said return line (206; 217) to cool said fuel before being returned to said fuel tank. a compressor section, a combustor, and a turbine section; and an associated fluid to be cooled, an associated fluid to be heated, and a transcritical vapor cycle for heating said fluid to be heated and for cooling said fluid to be cooled, said transcritical vapor cycle including a gas cooler, in which said fluid to be heated is heated by a refrigerant in said transcritical vapor cycle and an evaporator heat exchanger at which said fluid to be cooled is cooled by the refrigerant in said transcritical vapor cycle, a compressor upstream of said gas cooler for compressing the refrigerant to a pressure above a critical point for the refrigerant, and an expansion device for expanding the refrigerant downstream of said gas cooler, with said evaporator heat exchanger being downstream of said expansion device, and such that the refrigerant passing through said gas cooler to heat said fluid to be heated is generally above the critical point. wherein said fluid to be heated is fuel, characterised in that: said fuel to be heated is fuel passing to said combustor via a supply line, wherein the gas turbine engine further comprises a fuel system comprising said supply line, a return line, at least one return line heat exchanger, and a fuel tank, wherein said return line (206; 217) is downstream of said gas cooler for returning fuel to said fuel tank and said at least one return line heat exchanger (208, 214; 222, 224) is provided on said return line (206; 217) to cool said fuel before being returned to said fuel tank. said fuel to be heated is fuel passing to said combustor via a supply line, wherein the gas turbine engine further comprises a fuel system comprising said supply line, a return line, at least one return line heat exchanger, and a fuel tank, wherein said return line (206; 217) is downstream of said gas cooler for returning fuel to said fuel tank and said at least one return line heat exchanger (208, 214; 222, 224) is provided on said return line (206; 217) to cool said fuel before being returned to said fuel tank.