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Modulation is a process used in communications to alter a carrier signal in accordance with an information signal. The baseband signal carries the message, and the carrier provides a stable, high-frequency reference suitable for transmission over a physical medium. The modulated signal can then be transmitted through radio waves, copper wires, optical fiber, or other channels. At the receiver, a demodulator extracts the original information from the modulated carrier.

Modulation types fall into two broad categories: analog modulation and digital modulation. Analog modulation varies a property of the carrier continuously. Amplitude modulation (AM) changes the carrier amplitude in proportion to the message; frequency modulation (FM) varies the carrier frequency; phase modulation (PM) shifts the carrier phase. Digital modulation encodes discrete symbols onto the carrier: amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), and more advanced schemes such as quadrature amplitude modulation (QAM), which combines amplitude and phase changes to convey multiple bits per symbol.

Key considerations include bandwidth (the width of the spectrum used), power efficiency, and resilience to noise and distortion. Digital modulation is common in modern data communications because of efficient spectral use and error-correction compatibility, while analog modulation remains prevalent in traditional radio broadcasting.

Applications span radio and television broadcasting, mobile networks, satellite communications, and data links. Modulation is paired with demodulation to recover the original information, and it often involves additional techniques such as filtering, synchronization, and error correction.