A technique employed in telecommunications transmission systems whereby an electromagnetic signal (the modulating signal) is encoded into one or more of the characteristics of another signal (the carrier signal) to produce a third signal (the modulated signal), whose properties are matched to the characteristics of the medium over which it is to be transmitted. The encoding preserves the original modulating signal in that it can be recovered from the modulated signal at the receiver by the process of demodulation.

The main purpose of modulation is to overcome any inherent incompatibilities between the electromagnetic properties of the modulating signal and those of the transmission medium. Of primary importance in this respect is the spectral distribution of power in the modulating signal relative to the passband of the medium. Modulation provides the means for shifting the power of the modulating signal to a part of the frequency spectrum where the medium's transmission characteristics, such as its attenuation, interference, and noise level, are favorable.