Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 kHz to 30 kHz and wavelengths from 10 to 100 kilometres. Since there is not much bandwidth in this band of the radio spectrum, audio (voice) cannot be transmitted, and only low data rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations which broadcast time signals to set radio clocks, and for secure military communication. Since VLF waves penetrate about 40 meters into saltwater, they are used for military communication with submarines. Also known as the myriametre band or myriametre wave as the wavelengths range from one to ten myriametres (an obsolete metric unit equal to 10 kilometres).

Because of their large wavelengths, VLF radio waves can refract around large obstacles and so are not blocked by mountain ranges, and can propagate as a ground wave following the curvature of the Earth. The main mode of long distance propagation is an Earth-ionosphere waveguide mechanism. The Earth is surrounded by a conductive layer of ions in the atmosphere, the ionosphere D layer at 60 km altitude, which reflects VLF radio waves. The ionosphere and the conductive Earth, form a horizontal "duct" a few VLF wavelengths high, which acts as a waveguide confining the waves so they don't escape into space. The waves travel in a zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM (transverse magnetic) mode. VLF waves have very low path attenuation, 2-3 dB per 1000 km, with little of the "fading" experienced at higher frequencies, so they are used for long distance communication. Propagation distances of 5000 to 20000 km have been realized. However, atmospheric noise (sferics) is high in the band, including such phenomena as "whistlers", caused by lightning.