The challenging conditions at which gas turbine engines operate mean that careful management of component temperatures is necessary,in order to ensure component integrity and reasonable service life.Pressurised flows extracted from the compressor can be used for cooling,sealing and balancing of components throughout the engine.Cooling is typically necessary for some combustor and turbine components,and sealing flows may be needed to exclude high temperature gases.In addition to cooling and sealing requirements,there is also a need to balance thrust loads in an engine,to limit loads on bearings,arising from the pressure differentials across compressor and turbine disc assemblies.The diverse tasks of cooling,sealing and balancing are generally assigned to a system known as the internal air system or secondary flow system.This paper describes the technologies associated with this system for both industrial gas turbine engines and aero-engines,and the current state of the art and challenges in component and sub-system design.