The electrical machines used in aircraft differ greatly from normal industrial machines because they must have the lowest possible weight and bulk, but must be capable of operating over a wide range of ambient temperature and air pressure at varying altitudes. Special materials and methods of construction are therefore used in order to permit a high operating temperature and a high specific loading at a high speed.Machines both for direct current and alternating current are considered and are classified as generators, motors and converting machines. The well-established engine-driven blast-cooled d.c. generator receives a good deal of attention. The use of high-temperature insulation, brazed connections, impregnated brushes, high-permeability magnetic materials and other special features are discussed in some detail. D.C. motors, while offering more variety, give rise to fewer difficult design problems.Alternating-current generators may run over a wide speed range, as in many present-day installations in which most of the output is rectified in order to provide a d.c. supply, or they may be driven at constant speed from a variable-ratio gear coupled to the main engine or from a separate turbine. The insulation and constructional problems are similar to those of the d.c. generator. A.C. motors, inverters and rotary transformers introduce their own problems in relation to starting, regulation and other requirements.The paper attempts to give a general picture of the present state of development and to indicate the ways in which further advances are likely to be made in the future.