An aircraft environmental control system includes a bleed air input and a RAM air input, heat exchanger means for receiving bleed air from the bleed air input and RAM air from the RAM air input and using the RAM air to cool the bleed air, and ejector arranged to receive bleed air from the bleed air input at a nozzle shaped to reduce the pressure of the received bleed air such as to create a low pressure area in the ejector. The ejector has a port arranged such that ambient air is drawn into the ejector due to the low pressure area in the ejector. The ambient air is mixed with bleed air to provide mixed air that is further pressurised and conditioned and combined with the cooled bleed air provided to the aircraft.