An integrated approach to the understanding of charge-controlled electronic devices is presented. Although only vacuum triodes and diffusion-type transistors are discussed in detail, the methods suggested are also applicable to gas-filled and multi-electrode vacuum structures, to surface-barrier and to drift-type transistors, and to space-charge-limited solid-state devices. The treatment is tutorial in nature, and begins with the development of general equations of current flow applicable in any medium. The principles of charge-controlled devices are then summarized, and a general functional relationship between the total charge in transit and the transit time is developed. These results are then applied in turn to vacuum and semiconductor diodes and triodes to derive in a remarkably simple and consistent manner the salient features of their operation. ‘Ideal’ vacuum triode and transistor structures are first discussed, and the voltage and current amplification factors are then introduced as arbitrary parameters to account for practical departures from ideality. Specific results obtained are the d.c. characteristics and incremental equivalent circuits for each device. The model established for the transistor is identical with the hybrid-π circuit due to Giacoletto, and both low- and high-level injection conditions are included. Finally, it is suggested that the transistor collector saturation current with open base is a more fundamental quantity than that with open emitter, and the temperature dependence of the base-emitter voltage is shown to be linear at any injection level. Throughout, emphasis is on the principles involved and on the method of approach, and a particular effort is made to present the development of the vacuum and the semiconductor devices in a completely analogous manner.