The paper records the development of the generation of d.c. power in aircraft by windmill- and engine-driven generators, from its inception to the present day. The developments prior to the last decade are covered only very briefly.Features of generator design which have called for special consideration, or have been the subject of failure, are discussed. Sections are devoted to the choice of speed range, brush wear at high altitude, systems of ventilation, design of end-frames, and bearing failure. Some mention is made of generators designed for power supply to radio equipment, as distinct from general power services. These generators, which include high-voltage d.c. machines and high-frequency a.c. machines, have been combined with low-voltage d.c. machines, both in tandem and with a common magnet system.The latter part of the paper deals with voltage regulation, which is inseparable from the preceding part, in so far as generator design is influenced by the system adopted; self-regulating generators of various types have been used from time to time in the past. The scope does not permit more than brief mention of the design of the several types of regulator which have been used, including Tirrill and carbon-pile types.The paper shows how the control of the system voltage has been conditioned by the inclusion of an accumulator, and how it has not been possible to devise a system which gives the constant line voltage required for current-using devices, and, at the same time, permits of adequate control of accumulator charging current. Systems of paralleling, and their effect on line voltage, are also described.The electrical equipment of lighter-than-air craft is not included in this paper.