The general requirements for the complete air conditioning of aircraft are discussed in the light of the complete system concept. The author takes into consideration safety, differential pressure, weight saving, power and air supply, passenger comfort, cooling and humidity. Particular systems are then described and there is a section on the test equipment required for the laboratory testing of air conditioning equipment. Cooling systems are taken first and divided into the air cycle system embodying bootstrap, turbine fan, and regenerative applications of cold air units–the vapour cycle system employing a boiling tank and that using proprietary refrigerants; properties of liquid refrigerants are discussed. Regulation of cabin temperature, air flow, humidity, pressure and oxygen is done by control systems and the equipment used is described. Four appendices give (1) suggested detailed requirements for air conditioning equipment and user requirements, (2) sample data and calculations for air conditioning a 100-seater civil transport, (3) some notes on the definition of refrigeration terms and (4) data on pressure losses in aircraft ducts.