The brochure popularly describes the main physical phenomena necessary to understand the principle of operation of a tunnel diode, describes the parameters and characteristics of tunnel diodes, provides elementary information on the technology of their manufacture. The principles of building typical circuits (generator, amplifier, multivibrator, flip-flop) on tunnel diodes are also considered. The brochure is intended for a wide range of amateur radio operators who want to get an idea of ​ ​ the principle of operation and application of tunnel diodes.