A cathode ray tube (CRT) is a vacuum tube containing one or more electron guns whose beams are manipulated to display images on a phosphor screen.

These images may show electrical waveforms (oscilloscope), images (television set, computer monitor), radar targets, or other phenomena.

The CRT on a television set is commonly called a picture tube.

CRTs are also used as memory devices, in which case the screen is not intended for viewing by the observer. A German physicist working at the Strasbourg Institute of Physics (from 1918-1895) developed the first cold cathode tube with magnetic beam deflection.

He used a phosphor-coated mica diaphragm to create a visible spot. It was originally based on the Crooks deflection tube developed by Franz Müller. Brown used this tube as an indicator tube to visualize alternating currents and described it in 1897, in fact the first oscilloscope. Harris J. Ryan introduced a similar tube in 1903 in the United States as an alternative to the alternating current wave, known as the Brown-Ryan tube.

The first idea of using cathode rays (Brown tube) for television was proposed by Campbell Swinton of Scotland in 1908. The French Rinnox and the Russian Rosing also experimented with the Braun tube for the same reason, as did Blain and Holok, who used cathode rays for their television receiver and demonstrated it at Malmaison in 1928.