The FM-transmitter plugs into the audio output of audio devices and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car or portable radios. Most devices on the market can broadcast on any FM frequency from 76.0 to 108.0 MHz (or 87.9 to 107.9 in the US). Some lower-cost transmitters are hard-wired to the educational band (87.7-91.9), or a certain other smaller range of frequencies.

FM transmitters are either battery driven or use the cigarette lighter socket in cars. They are typically used with portable audio devices such as MP3, CD players or iPod.