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 . <EOS>