An aircraft environmental control system comprising a fresh air input and a RAM air input, heat exchanger means for receiving fresh air from the fresh air input and RAM air from the RAM air input and using the RAM air to cool the fresh air, and means for providing the cooled fresh air to a cabin of the aircraft, the system further comprising means for feeding back exhaust air emitted from the interior of the aircraft to combine with the RAM air to further cool the fresh air, whereby the heat exchanger means comprises a main heat exchanger and a primary heat exchanger; wherein the main heat exchanger comprising a main heat exchanger cooling fluid inlet, a main heat exchanger cooling fluid outlet and a main heat exchanger bleed air inlet, the cooling fluid working to cool bleed air in the main heat exchanger.