Tool is an American progressive rock band, formed in 1990 in Los Angeles, California, when drummer Danny Carey joined the rehearsal of his neighbor, singer Maynard James Keenan, guitarist Adam Jones and bassist Paul d'Amour, when nobody else would show up. His decision proved to be a stroke of luck when the band turned out to become a highly successful act, "introducing dark, vaguely underground metal to the preening pretentiousness of art rock" - most notably due to their influential third release, Ænima (1996).

They have gained appreciation and critical praise for a complex and ever-evolving sound, that ranges from "slam and bang" heavy metal on their first release to more progressive influenced songwriting on Lateralus (2001) which "in another era [...] would have been considered progressive rock, ten tons of impressive pretension." Their overall sound has been described as "grinding, post-Jane's Addiction heavy metal" as well as "a primal sound as distinct as it is disturbing" - most simplified categorizations of the band's genre are often dismissed (see: Genre and influences). They are known for addressing philosophical and spiritual issues in their lyrics, such as evolution and Jungian psychology ("Forty-Six & 2"), organized religion ("Opiate") and transcendence ("Lateralus"), as well as for songs that feature "complex rhythm changes, haunting vocals, and an onslaught of changes in dynamics" which often result in a greater-than-average track length.

Their music has been influenced by King Crimson, Led Zeppelin, Pink Floyd, Rush, early Genesis, early Yes, and Meshuggah. In turn, Tool's music has been deemed influential by critics and fellow artists alike.