A synthesizer (also synthesiser or synth) is an electronic musical instrument that generates audio signals. Synthesizers typically create sounds by generating waveforms through methods including subtractive synthesis, additive synthesis and frequency modulation synthesis. These sounds may be altered by components such as filters, which cut or boost frequencies; envelopes, which control articulation, or how notes begin and end; and low-frequency oscillators, which modulate parameters such as pitch, volume, or filter characteristics affecting timbre. Synthesizers are typically played with keyboards or controlled by sequencers, software or other instruments, and may be synchronized to other equipment via MIDI.

Synthesizer-like instruments emerged in the United States in the mid-20th century with instruments such as the RCA Mark II, which was controlled with punch cards and used hundreds of vacuum tubes. The Moog synthesizer, developed by Robert Moog and first sold in 1964, is credited for pioneering concepts such as voltage-controlled oscillators, envelopes, noise generators, filters, and sequencers. In 1970, the smaller, cheaper Minimoog standardized synthesizers as self-contained instruments with built-in keyboards, unlike the larger modular synthesizers before it.

In 1978, Sequential Circuits released the Prophet-5, which used microprocessors to allow users to store sounds for the first time. MIDI, a standardized means of synchronizing electronic instruments, was introduced in 1982 and remains an industry standard. The Yamaha DX7, launched in 1983, was a major success and popularized digital synthesis. Software synthesizers now can be run as plug-ins or embedded on microchips. In the 21st century, analog synthesizers returned to popularity with the advent of cheaper manufacturing and the increasing popularity of synthwave music starting in the 2010s.

Synthesizers were initially viewed as avant-garde, valued by the 1960s psychedelic and countercultural scenes but with little perceived commercial potential. Switched-On Bach (1968), a bestselling album of Bach compositions arranged for synthesizer by Wendy Carlos, took synthesizers to the mainstream. They were adopted by electronic acts and pop and rock groups in the 1960s and 1970s and were widely used in 1980s music. Sampling, introduced with the Fairlight synthesizer in 1979, has influenced genres such as electronic and hip hop music. Today, the synthesizer is used in nearly every genre of music and is considered one of the most important instruments in the music industry. According to Fact in 2016, "The synthesizer is as important, and as ubiquitous, in modern music today as the human voice."

As electricity became more widely available, the early 20th century saw the invention of electronic musical instruments including the Telharmonium, Trautonium, ondes Martenot and theremin. In the late 1930s, the Hammond Organ Company built the Novachord, a large instrument powered by 72 voltage-controlled amplifiers and 146 vacuum tubes. In 1948, the Canadian engineer Hugh Le Caine completed the electronic sackbut, a precursor to voltage-controlled synthesizers, with keyboard sensitivity allowing for vibrato, glissando, and attack control.

In 1957, Harry Olson and Herbert Belar completed the RCA Mark II Sound Synthesizer at the RCA laboratories in Princeton, New Jersey. The instrument read punched paper tape that controlled an analog synthesizer containing 750 vacuum tubes. It was acquired by the Columbia-Princeton Electronic Music Center and used almost exclusively by Milton Babbitt, a composer at Princeton University.

The authors of Analog Days define "the early years of the synthesizer" as between 1964 and the mid-1970s, beginning with the debut of the Moog synthesizer. Designed by the American engineer Robert Moog, the instrument was a modular synthesizer system composed of numerous separate electronic modules, each capable of generating, shaping, or controlling a sound depending on how each module is connected to other modules by patch cables. Moog developed a means of controlling pitch through voltage, the voltage-controlled oscillator. This, along with Moog components such as envelopes, noise generators, filters, and sequencers, became standard components in synthesizers.

Around the same period, the American engineer Don Buchla created the Buchla Modular Electronic Music System. Instead of a conventional keyboard, Buchla's system used touchplates which transmitted control voltages depending on finger position and force. However, the Moog's keyboard made it more accessible and marketable to musicians, and keyboards became the standard means of controlling synthesizers. Moog and Buchla initially avoided the word synthesizer for their instruments, as it was associated with the RCA synthesizer; however, by the 1970s, it had become the standard term.