A flash tube is a gas-filled device designed to produce a brilliant flash of light when it is used to short-circuit a charged capacitor. The discharge is characterized by a high output of radiation in the infrared and ultra-violet as well as in the visible regions.A brief historical review is followed by an outline of the general construction of modern flash tubes. Their electrical characteristics are given together with curves showing current, voltage and power against time. The effect of the tube parameters on the complete circuit characteristics are examined.Some measurements of light intensity, luminous energy and efficiency are given, and the development of the typical high-current arc spectra of xenon under flash-tube conditions is illustrated by a series of spectrograms.Circuits for the control of these tubes are described, suitable for both isolated-flash and repetition-flash applications.The concluding Sections deal with a number of recent applications and with an examination of the possibilities of the flash tube as an aid to the examination of high-speed phenomena, both photographically and visually.