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; characterised in that: the gas turbine engine comprises a fuel supply system and a fuel tank, wherein said fluid to be cooled is fuel returning from the fuel supply system to the fuel tank; and the fuel supply system comprises main fuel line including a first valve and a first heat exchanger configured to heat the fuel, and a bypass line bypassing the first valve and the first heat exchanger, wherein downstream of the heat exchanger the fuel returns to the main fuel line, the main fuel line having there a second valve configured to meter flow of the fuel to the combustor, and the bypass line comprises a branch and a third valve configured to control a flow of the fuel into the branch which is configured to return the fuel to the fuel tank via a second heat exchanger of the fuel supply system configured for being provided with ram air, a third heat exchanger of the fuel supply system configured for being provided with air cycle machine air, and said evaporator heat exchanger. 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; characterised in that: the gas turbine engine comprises a fuel supply system and a fuel tank, wherein said fluid to be cooled is fuel returning from the fuel supply system to the fuel tank; and the fuel supply system comprises main fuel line including a first valve and a first heat exchanger configured to heat the fuel, and a bypass line bypassing the first valve and the first heat exchanger, wherein downstream of the heat exchanger the fuel returns to the main fuel line, the main fuel line having there a second valve configured to meter flow of the fuel to the combustor, and the bypass line comprises a branch and a third valve configured to control a flow of the fuel into the branch which is configured to return the fuel to the fuel tank via a second heat exchanger of the fuel supply system configured for being provided with ram air, a third heat exchanger of the fuel supply system configured for being provided with air cycle machine air, and said evaporator heat exchanger. the gas turbine engine comprises a fuel supply system and a fuel tank, wherein said fluid to be cooled is fuel returning from the fuel supply system to the fuel tank; and the fuel supply system comprises main fuel line including a first valve and a first heat exchanger configured to heat the fuel, and a bypass line bypassing the first valve and the first heat exchanger, wherein downstream of the heat exchanger the fuel returns to the main fuel line, the main fuel line having there a second valve configured to meter flow of the fuel to the combustor, and the bypass line comprises a branch and a third valve configured to control a flow of the fuel into the branch which is configured to return the fuel to the fuel tank via a second heat exchanger of the fuel supply system configured for being provided with ram air, a third heat exchanger of the fuel supply system configured for being provided with air cycle machine air, and said evaporator heat exchanger.