Rock music is a loosely defined genre of popular music that entered the mainstream in the mid 1950s. It has its roots in 1940s and 1950s rhythm and blues, country music and also drew on folk music, jazz, and classical music.

The sound of rock often revolves around the electric guitar or acoustic guitar, and it uses a strong back beat laid down by a rhythm section of electric bass guitar, drums, and keyboard instruments such as organ, piano, or, since the 1970s, synthesizers. Along with the guitar or keyboards, saxophone and blues-style harmonica are sometimes used as soloing instruments. In its "purest form", it "has three chords, a strong, insistent back beat, and a catchy melody."

In the late 1960s and early 1970s, rock music developed different subgenres. When it was blended with folk music it created folk rock, with blues to create blues-rock and with jazz, to create jazz-rock fusion. In the 1970s, rock incorporated influences from soul, funk, and latin music. Also in the 1970s, rock developed a number of subgenres, such as soft rock, glam rock, heavy metal, hard rock, progressive rock, and punk rock. Rock subgenres that emerged in the 1980s included New Wave, hardcore punk and alternative rock. In the 1990s, rock subgenres included grunge, Britpop, indie rock, and nu metal.

A group of musicians specializing in rock music is called a rock band or rock group. Many rock groups consist of an electric guitarist, lead singer, bass guitarist, and a drummer, forming a quartet. Some groups omit one or more of these roles and/or utilize a lead singer who plays an instrument while singing, sometimes forming a trio or duo; others include additional musicians such as one or two rhythm guitarists and/or a keyboardist. More rarely, groups also utilize stringed instruments such as violins or cellos, or horn sections of saxophones, trumpets or trombones.

Rock and roll evolved in the United States in the late 1940s and early 1950s, and quickly spread to the rest of the world. Its immediate origins lay in a mixing together of various popular musical genres of the time, including rhythm and blues, gospel music, and country and western. In 1951, Cleveland, Ohio disc jockey Alan Freed began playing rhythm and blues music for a multi-racial audience, and is credited with first using the phrase "rock and roll" to describe the music.

There is much debate as to what should be considered the first rock & roll record. One leading contender is "Rocket 88" by Jackie Brenston and his Delta Cats (in fact, Ike Turner and his band The Kings of Rhythm), recorded by Sam Phillips for Sun Records in 1951. Four years later, Bill Haley's "Rock Around the Clock" (1955) became the first rock and roll song to top Billboard magazine's main sales and airplay charts, and opened the door worldwide for this new wave of popular culture. Rolling Stone magazine argued in 2004 that "That's All Right (Mama)" (1954), Elvis Presley's first single for Sun Records in Memphis, was the first rock and roll record. But, at the same time, Big Joe Turner's "Shake, Rattle & Roll", later covered by Haley, was already at the top of the Billboard R&B charts. Other artists with early rock and roll hits included Chuck Berry, Bo Diddley, Fats Domino, Little Richard, Jerry Lee Lewis, and Gene Vincent.

The 1950s saw the growth in popularity of the electric guitar, and the development of a specifically rock and roll style of playing through such exponents as Berry, Link Wray, and Scotty Moore. It also saw major developments in recording technology such as multitrack recording developed by Les Paul, the electronic treatment of sound by such innovators as Joe Meek, and the Wall of Sound productions of Phil Spector. All these developments were important influences on later rock music.

The social effects of rock and roll were worldwide and massive. Far beyond simply a musical style, rock and roll influenced lifestyles, fashion, attitudes, and language. In addition, rock and roll may have helped the cause of the civil rights movement because both African American teens and white American teens enjoyed the music. However, by the early 1960s, much of the initial musical impetus and social radicalism of rock and roll had become dissipated, with the growth of teen idols, an emphasis on dance crazes, and the development of lightweight teenage pop music. The early 60's did see the rise of the Motown sound. From 1961 to 1971, Motown had 110 top 10 hits, and artists such as Stevie Wonder, Marvin Gaye, The Supremes, The Four Tops, and The Jackson 5, were all signed to Motown labels. All five of these Motown artists have been inducted into the Rock and Roll Hall of Fame.

In the United Kingdom, the trad jazz movement brought visiting blues music artists to Britain. Lonnie Donegan's 1955 hit "Rock Island Line" was a major influence, and helped to develop the trend of skiffle music groups throughout the country, including John Lennon's The Quarrymen. Britain developed a major rock and roll scene, without the race barriers which kept "race records" or rhythm and blues separate in the US.

Cliff Richard had the first British rock 'n' roll hit with "Move It", effectively ushering in the sound of British rock. At the start of the 1960s, his backing group The Shadows was one of a number of groups having success with instrumentals. While rock 'n' roll was fading into lightweight pop and ballads, British rock groups at clubs and local dances, heavily influenced by blues-rock pioneers like Alexis Korner, were starting to play with an intensity and drive seldom found in white American acts.

By the end of 1962, the British rock scene had started with beat groups like the Beatles drawing on a wide range of American influences including soul music, rhythm and blues and surf music. Initially, they reinterpreted standard American tunes, playing for dancers doing the twist, for example. These groups eventually infused their original rock compositions with increasingly complex musical ideas and a distinctive sound. In mid-1962 The Rolling Stones started as one of a number of groups increasingly showing blues influence, along with The Animals and The Yardbirds.

British rock broke in the United States in January 1964 with the success of the Beatles. "I Want to Hold Your Hand" was the band's first number-one hit on the Billboard Hot 100 chart, starting the British Invasion of the American music charts. The song entered the chart on January 18 1964 at number 45 before it became the number one single for 7 weeks and went onto last a total of 15 weeks in the chart. It also held the top spot in the United Kingdom charts. A million copies of the single had already been ordered on its release. "I Want to Hold Your Hand" became The Beatles' best-selling single worldwide. Their first appearance on the Ed Sullivan Show February 9 is considered a milestone in American pop culture. The broadcast drew an estimated 73 million viewers, at the time a record for an American television program. The Beatles went on to become the biggest selling rock band of all time and they were followed by numerous British bands.

In late 1964, The Kinks, The Who and The Pretty Things represented the new Mod style. The Rolling Stones broke in late 1964 as well. Their first international number-1 hit was "(I Can't Get No) Satisfaction", recorded in May 1965 during the band's third North American tour. Released as a US single in June 1965, it spent four weeks at the top of the charts there, and established the Stones as a worldwide premier act. Towards the end of the decade, British rock groups began to explore psychedelic musical styles that made reference to the drug subculture and hallucinogenic experiences.

From the late 1950s, increasing numbers of groups were formed across the USA by young and enthusiastic musicians, often rehearsing in their parents' garages, performing at local dances and shows, and recording and releasing their own songs and covers, often on small local labels. By 1963, garage band singles were creeping into the national charts in greater numbers, including the Kingsmen (Portland), Paul Revere and the Raiders (Boise), the Trashmen (Minneapolis) and the Rivieras (South Bend, Indiana).

The British Invasion encouraged a further wave of imitators. Some music from this trend is included in the compilation album Nuggets. Some of the better known bands of this genre include The Sonics, Question Mark & the Mysterians,[The Standells] and the[New Colony Six]-known as the U.S. answer to the British invasion].

The rockabilly sound influenced a wild, mostly instrumental sound called surf music, though surf culture saw itself as a competing youth culture to rock and roll. This style, exemplified by Dick Dale and The Surfaris, featured faster tempos, innovative percussion, and reverb- and echo-drenched electric guitar sounds. In the UK at the same time, popular instrumental groups included The Shadows. Other West Coast bands, such as The Beach Boys and Jan and Dean slowed the tempos down and added lush harmony vocals to create what became known as the "California Sound".

In the late 1950s the US beatnik counterculture was associated with the wider anti-war movement building against the threat of the atomic bomb, notably CND in Britain. Both were associated with the jazz scene and with the growing folk song movement.

Pop rock is a mix of pop music and rock music that uses catchy pop style, with light lyrics over top of guitar-based songs. There are varying definitions of the term, ranging from it being classed as a slower and mellower form of rock music to a subgenre of pop music. Scholars have noted that pop and rock are usually depicted as opposites; the detractors of pop often deride it as a slick, commercial product along with advocates of rock who claim that rock music is a more authentic, sincere form of music.

The textbook American Popular Music defines pop rock as an "upbeat variety of rock music represented by artists such as The Beatles, Elton John, Paul McCartney, The Everly Brothers, Wings (Paul McCartney and Wings), Rod Stewart, Chicago, and Peter Frampton.

The folk scene was made up of folk music lovers who liked acoustic instruments, traditional songs, and blues music with a socially progressive message. The folk genre was pioneered by Woody Guthrie. Bob Dylan came to the fore in this movement, and his hits with Blowin' in the Wind and Masters of War brought "protest songs" to a wider public.

Inspired by the success of the Beatles to mix folk and rock, Roger McGuinn had already been playing Beatles songs acoustically in Los Angeles folk clubs when Gene Clark approached him to form an act. The Byrds, playing Bob Dylan's Mr. Tambourine Man, helped start the trend of folk rock, and helped stimulate the development of psychedelic rock. Dylan continued, with his "Like a Rolling Stone" becoming a US hit single. Neil Young's lyrical inventiveness and wailing electric guitar attack created a variation of folk rock. Other folk rock artists include Simon & Garfunkel, Joan Baez, The Mamas & the Papas, Joni Mitchell, Bobby Darin and The Band.In Britain, Fairport Convention began applying rock techniques to traditional British folk songs, followed by groups such as Steeleye Span, Lindisfarne, Pentangle, and Trees. Alan Stivell in Brittany had the same approach.

Psychedelic music's LSD-inspired vibe began in the folk scene, with the Holy Modal Rounders popularizing the term in 1964. With a background including folk and jug band music, bands like the Grateful Dead and Big Brother & the Holding Company became two famous bands of the genre. The Fillmore was a regular venue for groups like another former jug band, Country Joe and the Fish, and Jefferson Airplane. Elsewhere, The Byrds had a hit with Eight Miles High. The 13th Floor Elevators titled their album The Psychedelic Sounds of the 13th Floor Elevators. The music increasingly became associated with opposition to the Vietnam War.

In England, Pink Floyd had been developing psychedelic rock since 1965 in the underground culture scene. In 1966 the band Soft Machine was formed. Donovan had a folk music-influenced hit with Sunshine Superman, one of the early psychedelic pop records. In August 1966 The Beatles released their Revolver album, which featured psychedelia in "Tomorrow Never Knows" and in "Yellow Submarine", along with the memorable album cover. The Beach Boys responded in the U.S. with Pet Sounds. From a blues rock background, the British supergroup Cream debuted in December, and Jimi Hendrix became popular in Britain before returning to the US.

The psychedelic scene took off in 1967, with The Doors and Jefferson Airplane releasing drug-themed LPs and the Beatles releasing Sgt. Pepper's Lonely Hearts Club Band and The Rolling Stones released Their Satanic Majesties Request. As the Summer of Love reached its peak, the Monterey Pop Festival featured Jefferson Airplane and introduced Janis Joplin and Jimi Hendrix. The culmination of the socially unifying trend was the rock festivals such as Woodstock in 1969. The Paisley Underground bands of Los Angeles epitomized the role played by 1960s psychedelia and folk-rock in American New Wave.

Glam rock emerged out of the English Psychedelic and art rock scene of the late 1960s, defined by artists such as T. Rex, Roxy Music, Steve Harley and Cockney Rebel, and David Bowie, also with origins in the theatrics of groups such as The Cockettes, performers such as Lindsay Kemp, and acts such as Syd Barrett's Pink Floyd (as represented in David Bowie's cover of See Emily Play) and Eddie Cochran (as represented by T. Rex's cover of Summertime Blues). The commonly accepted origin of Glam rock was when Tyrannosaurus Rex - a band produced by Tony Visconti and championed by the legendary John Peel - frontman/singer Marc Bolan changed the band's name to T. Rex, releasing the number 1 UK single Ride A White Swan in December 1970, ushering in Glam rock and the band as a pop phenomenon. Following soon after were other notable acts such as Slade and Roxy Music, and eventually David Bowie's Ziggy Stardust persona, who brought Glam rock its relatively novel and modest popularity in America, and leading to American artists such as Lou Reed, Iggy Pop, New York Dolls, Jobriath, and Alice Cooper adopting Glam or Glam-influenced styles.

Glam itself was a nostalgic mesh of various styles, both visual art and music, ranging from 1930s Hollywood glamor, to 1950s pin-up sex appeal and rock n' roll teenage rebellion, to pre-war Cabaret theatrics, to Victorian literary and Symbolist styles, to ancient and occult mysticism and mythology (such as Bowie's references to Aleister Crowley's "starman" in his song of the same name, and themes of reincarnation and self-invention in T. Rex's Cosmic Dancer). Glam is most noted for its sexual and gender ambiguity and androgyny, and use of theatrics.

Throughout glam rock's popularity, many bubble-gum acts - such as Elton John, Slade, Gary Glitter, and Alvin Stardust - adopted raunchier and more sexual takes on Glam style. Other previously famous acts such as The Rolling Stones and Lou Reed re-invented themselves in a glam fashion, often to great success (including Reed's biggest hit single, "Walk On the Wild Side"). However, glam's success in America was modest at best, with artists such as T. Rex and Roxy Music having only a fraction of the success they had in the UK. However, glam went on to influence many other genres, including punk, new wave, goth, jangle pop, college rock, and grunge, with artists as diverse as Siouxsie Sioux, Johnny Rotten, Billy Corgan, Peter Murphy (whose band Bauhaus covered T. Rex's Telegram Sam and Bowie's Ziggy Stardust), and Adam Ant citing glam artists as key influences. Glam has since enjoyed sporadic modest revivals through bands such as Chainsaw Kittens and The Darkness.

Progressive rock bands went beyond the established rock music formulas by experimenting with different instruments, song types, and musical forms. Some bands such as The Beatles, Pink Floyd, The Moody Blues and Procol Harum experimented with new instruments including wind sections, string sections, and full orchestras. Many of these bands moved well beyond the formulaic three-minute rock songs into longer, increasingly sophisticated songs and chord structures. With inspiration from these earlier artists, referred to as "proto-prog", it flowered into its own genre, initially based in the UK, after King Crimson's 1969 genre-defining debut album, In the Court of the Crimson King.

Progressive rock bands pushed "rock's technical and compositional boundaries" by going beyond the standard rock or popular verse-chorus-based song structures. Additionally, the arrangements often incorporated elements drawn from classical, jazz, and world music. Instrumentals were common, while songs with lyrics were sometimes conceptual, abstract, or based in fantasy. Progressive rock bands sometimes used "concept albums that made unified statements, usually telling an epic story or tackling a grand overarching theme." Progressive rock came into most widespread use around the mid-1970s. Few bands achieved major mainstream success, but large cults followed many of the groups. Pink Floyd, Supertramp, Yes, Emerson, Lake and Palmer, Marillion, Rush, Jethro Tull, Genesis, and a few less notable others were able to work in hit singles to their otherwise complex and untraditional albums to garner a larger audience.

With the advent of punk rock in the late 1970s, critical opinion in England moved toward a simpler and more aggressive style of rock, with progressive bands increasingly dismissed as pretentious and overblown, ending progressive rock's reign as one of the leading styles in rock. This was part of a wider commercial turn in popular music in the second half of the 1970s, during which many funk or soul bands switched to disco, and smooth jazz gained popularity over jazz fusion.

However, established progressive bands still had a strong fan base; Rush, Genesis, ELP, Yes, Queen, and Pink Floyd all regularly scored Top Ten albums with massive accompanying tours, the largest yet for some of them. From 1976 to 1980, heavy metal pioneers Led Zeppelin would display a minor prog-influence on their Presence[citation needed] and In Through the Out Door albums.

By 1979, by which time punk had mutated into new wave, Pink Floyd released their rock opera The Wall, one of the best selling albums in history. Many bands which emerged in the aftermath of punk, such as Siouxsie and The Banshees, Cabaret Voltaire, Ultravox, Simple Minds, and Wire, all showed the influence of prog, as well as their more usually recognised punk influences.