Aircraft air‐conditioning has developed over a period of about thirty years. In that time the general appearance of the hardware has changed surprisingly little. The components used remain the same as in the earliest application; air to air heat exchangers, high speed expansion turbines, valves and water extraction devices combined into what is now referred to as “Environmental Control System”. Although the basic form of the components has changed little, the arrangement of the components within the system and their method of operation have changed considerably and we are now on the threshold of even more radical changes.