A turbofan gas turbine engine includes, in axial flow sequence, a heat exchanger module, a fan assembly, a compressor module, and a turbine module. The fan assembly includes a plurality of fan blades defining a fan diameter, and the heat exchanger module is in fluid communication with the fan assembly by an inlet duct. The heat exchanger module includes a plurality of heat transfer elements for transfer of heat from a first fluid contained within the heat transfer elements to an airflow passing over a surface of the heat transfer elements prior to entry of the airflow into an inlet to the fan assembly. At full-power condition, the engine produces a maximum thrust T (N), the heat exchanger module transfers a maximum heat rejection H (W) from the first fluid to the airflow, and a Heat Exchanger Performance parameter PEX (W/N) defined as PEX=H/T is 0.4 to 6.0.