Extreme metal is a loosely defined umbrella term for a number of related heavy metal music subgenres that have developed since the early 1980s. It has been defined as a "cluster of metal subgenres characterized by sonic, verbal, and visual transgression", and refers to metal that is harsher, heavier, more aggressive and less commercialized than other subgenres. It is generally underground music. The term usually includes thrash metal, death metal, black metal and doom metal. Some definitions do not recognise doom metal, or consider that only some subgenres of it are extreme. Most extreme metal styles have very fast tempos and originally took inspiration from hardcore punk.

"Extreme metal" generally refers to heavy metal that is extreme in terms of its sound, instrumentation, vocals, lyrics, and imagery. Extreme metal usually has heavily distorted guitars, harsh vocals (such as shouting, screaming and death growls), and complex drumming (such as double bass drumming and blast beats). Most extreme metal styles have very fast tempos (usually between 150 and 250 beats per minute). The exception is doom metal, which instead often takes heaviness and slowness to extremes. Most extreme metal eschews conventional melody and verse-chorus-verse song structure. It also tends to omit the blues element of traditional heavy metal. Joel McIver wrote that extreme metal tends to be faster, harsher, heavier or more aggressive than traditional heavy metal.

Extreme metal's lyrics and imagery typically deal with dark themes such as death, killing, war, horror and the occult. They are often anti-Christian and may include references to Satanism or Paganism. A small number of extreme metal bands reference Nazi Germany, for example Slayer and Marduk.

According to ethnographer Keith Kahn-Harris, the defining characteristics of extreme metal are clearly transgressive: the "extreme" traits noted above are all meant to violate or transgress cultural, artistic, social or aesthetic boundaries. Kahn-Harris says that extreme metal can sometimes sound "close to being a formless noise" to the uninitiated listener.

In general, extreme metal strives to remain inaccessible and unpalatable to 'mainstream' audiences. It is largely based around independent record labels and grassroots promotion. In most of the world, extreme metal does not receive much radio-play or achieve high chart positions.

The English band Venom laid the foundations of extreme metal with their first two albums Welcome to Hell (1981) and Black Metal (1982). Part of the new wave of British heavy metal, they played a sped-up and stripped-down style, and used Satanic and occult imagery. Venom and early extreme metal bands were influenced by the speed, distortion and aggression of hardcore punk bands like Discharge, the Exploited and Amebix.

The first thrash metal bands were inspired by the likes of Venom, Motörhead, the new wave of British heavy metal and hardcore punk. In 1983, Metallica released their debut album Kill 'Em All, which is seen as the first thrash metal record, and would eventually be certified triple platinum. A few months later, Slayer released their own thrash metal album Show No Mercy. These were "landmark releases characterized by speed, aggression, and an austere seriousness". Thrash was often called "speed metal" in the early 1980s, before the two terms became more defined.

Swedish band Bathory have been described as "the biggest inspiration for the Norwegian black metal movement of the early nineties". Their songs first appeared on the compilation Scandinavian Metal Attack in March 1984. Bathory's first and second albums respectively influenced black metal and death metal. Frontman Quorthon pioneered the shrieked vocals that later came to define black metal.