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Hard rock (also referred to as heavy rock) is a variation of rock and roll music which has its earliest roots in mid-1960s garage and psychedelic rock. It is typified by a heavy use of distorted electric guitars, bass guitar, keyboards and drums. The term "hard rock" is often used as an umbrella term for genres such as grunge or metal and in order to distinguish them from pop rock.

Hard rock is strongly influenced by blues music^{[citation needed]}; the most frequently used scale in hard rock is the pentatonic, which is a typical blues scale. Unlike traditional rock and roll (which takes elements of the "old" blues), hard rock incorporates elements of "British blues", a style of blues played with more modern instruments such as electric guitars, drums, keyboards and electric bass. A notable departure from traditional blues forms is that hard rock is seldom restricted to the I, IV, and V chords prevalent in twelve or sixteen bar blues, but includes other chords, typically major chords rooted on tones of the minor scale.

The term "hard rock" is often applied to many styles of rock music, their only common feature being that they deviate from pop rock, though this is generally incorrect. Two such examples are punk rock and grunge. Punk rock uses a faster tempo, less melody, fewer riffs (often using power chords), more aggression and anti-establishment lyrics.

The predominant instruments in hard rock are the electric guitar, bass guitar, and drums. The role of the guitarist is very prevalent in hard rock. Most hard rock bands comprise two different types of guitarist: lead guitarist and rhythm guitarist. The lead guitarist plays the solos, riffs and fills. Speed-enhancing techniques such as alternate picking, sweep picking and tapping, are used by hard rock lead guitarists to maximize the speed of their solos and riffs. The role of the rhythm guitarist is to complement the lead guitarist and provide rhythmic and harmonic accompaniment to the other instruments in the band. The bass guitarist and drummer's role are important to the structure of hard rock music; the bassline outlines the harmony of the music while the drums sustain the rhythm of the music.

The early 1990s were at first dominated by Guns N' Roses and Metallica. The multi-platinum releases of Metallica's "Metallica" (often referred to as "The Black Album") and Guns N' Roses' Use Your Illusion I and Use Your Illusion II in 1991 showcased this popularity. But the popularity of such bands waned, as their music and attitudes became more decadent and self-indulgent. In 1991 a new form of hard rock broke into the mainstream.

Grunge combined elements of hardcore punk and heavy metal into a dirty sound that made use of heavy guitar distortion, fuzz and feedback. Although most grunge bands had a sound that sharply contrasted mainstream hard rock (for example Nirvana, Mudhoney and L7), a minority (for example Pearl Jam, Mother Love Bone, Temple of the Dog and even Soundgarden) were more strongly influenced by much 1970s and 1980s rock and metal. However, all grunge bands shunned the macho, anthemic and fashion-focused style of hard rock at that time.

In the UK, bands like Swervedriver, Catherine Wheel and Ride demonstrated that guitar heroics could be incorporated into songs that lacked the often-misogynistic content of 1970s and 1980s hard rock bands. As the popularity of artists such as Metallica continued from the 1980s into the 1990s, some other bands had begun to fuse metal with a range of eclectic influences. These bands came to be known as alternative metal artists, a subset of alternative rock. Some, such as Primus, Red Hot Chili Peppers, Rage Against the Machine, Living Colour and White Zombie fused funk with metal styles, though most of these bands actually formed in the '80s. Faith No More/Mr. Bungle fused many genres with hard rock, ranging from rap music to soul. Helmet and The Afghan Whigs were also successful experimental hard rock bands. The Darkness's retro glam-metal influences helped propel them to the upper realms of the charts in the early 2000s, with the likes of Wolfmother. Towards the mid 2000s with new bands started to become mainstream, Jet, Wolfmother, White Stripes, The Answer, Glitterati, The Datsuns, Nineteenth Century and Punk influence Towers of London are some of the new rock bands which followed up from the Garage rock revival.

The biggest major hard rock bands of recent years however, have been supergroups Velvet Revolver and Audioslave. Audioslave consisted of Rage Against the Machine instrumentalists and former Soundgarden frontman Chris Cornell and was disbanded in 2007. Velvet Revolver is made up of ex-members of Guns N' Roses primarily,with vocalist Scott Weiland formerly of the Stone Temple Pilots, and the musicians have updated the sound of hard rock.^{[citation needed]} This has helped revive the glam metal scene (e.g. bands like Buckcherry, which Guns N' Roses Appetite for Destruction album is often credited with influencing).