Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden, in which new frequencies are created by combining or mixing two frequencies. Heterodyning is useful for frequency shifting signals into a new frequency range, and is also involved in the processes of modulation and demodulation. The two frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer. In the most common application, two signals at frequencies f₁ and f₂ are mixed, creating two new signals, one at the sum f₁ + f₂ of the two frequencies, and the other at the difference f₁ − f₂. These new frequencies are called heterodynes. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer. Heterodynes are closely related to the phenomenon of "beats" in music.

In 1901, Reginald Fessenden demonstrated the heterodyne detector as a method of making continuous wave radiotelegraphy signals audible. The detector did not see much application because stable local oscillators would not be available until the invention of the triode vacuum tube oscillator. In a 1905 patent, Fessenden states the frequency stability of his oscillator was one part per thousand.