Tool are among the most relevant figures of the grunge era. Their sinister and menacing brand of rock - "hard" bordering on "heavy" - followed on the success of Soundgarden and Metallica and wore these musical stigmata. But in Tool, the guitarist (Adam Jones) and singer (Maynard James Keenan) were often less important than the rhythm section, consisting of Danny Carey on drums and Paul D'Amour on bass. Keenan would grow in time, but early on it was the killer rhythms of his bandmates that gave intensity to their songs.

Formed in Los Angeles in May 1991, they introduced themselves with the EP Opiate (Zoo, 1992). Songs like Sweat blur the line between grunge, hardcore, and speed metal; but the best to be found here is Hush, a bleak 21st Century Schizoid Man (King Crimson) of grunge; and Cold And Ugly, a hypnotic, sweeping gallop in the vein of Whole Lotta Love (boasting the most effective solo on the record).
In spite of the musicians' technical prowess, none of the compositions live up to their ambitions, which are on par with Shakespearean tragedy: inspired by the "lachrymology" of philosopher Ronald Vincent, and more prosaically by the early Swans, Tool want to make existential music, all while using some of the most hedonistic sounds imaginable.

Undertow (Zoo Entertainment, 1993) fully exploits the fundamental oxymoron that represents the core of Tool's aesthetic program: deliver an existential message via hedonistic sounds. The resulting work looks apparently difficult, confused, obscure, disquieting, something more similar to a Freudian session than to a rock concert, with a concentrate of riffs serving a solemn and public dramaturgy. In fact, all the songs rely on tense, unstable, brutal dynamics, just like a theatrical tragedy.
The best scripts belong to the furious Undertow, a stylistic and emotional rollercoaster that climbs visceral heights only to fall into sterile abysses, and the bleak Intolerance, shaken by tribal Bo Diddley-esque beats in a black-mass atmosphere, with an incandescent instrumental coda. Prison Sex summarizes the idea in the interplay of two elements: a vibrant rhythm'n'blues shout and a southern-boogie guitar. They all recite the same plot: a bluesy hard-rock underpinned by a highly personal technique of guitar riffs, tempo shifts, collective bursts and emotional vocals, so as to shroud it in an oppressive ambience. The exhilarating seven-minute Bottom (circular bass riff, fractured guitar, roaring refrain, syncopated drums, long quiet intermezzo) is a sort of Led Zeppelin's Whole Lotta Love for the post-grunge generation. Crawl Away stages a relentless emotional crescendo that peaks with a breathless rhythmic cavalcade with the guitar whipping the drums.
Occasionally this praxis approached the manner of 1970s' glam-rock (the power-ballad Sober, the magniloquent Swamp Song).
The last two tracks, though (also the longest ones), demonstrated Tool's limitations when they stretched beyond their home turf.
Adam Jones' nuanced guitar playing (a far cry from heavy metal's traditional solos) paled in comparison with Paul d'Amour's growling bass.

Tool rapidly became one of the most beloved live acts. The album took a while to climb the charts, but eventually the singles Sober and Prison Sex turned the band into stars.

Paul D'Amour also played in the Replicants (Zoo, 1996) with Ken Andrews and Greg Edwards of Failure, but it's a band that only performed covers.

Aenima (Zoo Entertainment, 1996), featuring new bassist Justin Chancellor, presented Tool as mature composers and (relatively) austere performers, engaged in lengthy and convoluted suites (interleaved with brief experimental pieces). Aenima was, in many ways, a natural evolution from the previous work: the songs exhibit the same vanity of visceral eccentricity, but, instead of merely rumbling and whirring, they display an elegant vocal-guitar-rhythm dialectics.
Tool carefully balances the canon of grunge and the melodramatic structures of prog-rock. Stinkfist's epic/fatalistic tone and melodic/catastrophic dynamics evokes Jethro Tull's Aqualung, but the nervousness is all theirs: the song stops and seems to die countless times, regardless of the propulsive energy that boils underneath, but is always reborn more stately than before. The eight-minute Eulogy, which borrows the idea for the percussive intro from the Who's Magic Bus, is, musically speaking, an essay on uncertainty: not only do the riffs ebb and flow, they also sink in a swamp of loosely-organized passages. It doesn't always work: at ten minutes, Pushit is a bad case of bombastic progressive-rock gone berserk. The 14-minute colossus Third Eye is a (weak) exercise in chaos and randomness more than a cohesive song, although it boasts fine guitar work.
Tool's art is one of subtle contrasts and subdued antinomies. Existential despair and titanic will compete in Forty-Six And 2. Disgust and pain tear Hooker with a Penis apart. Aenema alternates between a colloquial lament and an emphatic Cream-like refrain. Consummate storytellers, the musicians of Tool turn each score into a wrestling match.
The sense of claustrophobia, which always was their trademark, envelops experimental tracks such as the industrial blues Die Eier Von Satarn and the electronic poem Ions like a poisonous fog.
The album is Keenan's satori and a sort of diary of his primal angst. It is telling that Tool consistently fail when they try to stretch out their songs. While they aim for the epic-length masterpiece, they seem incapable of maintaining focus and imagination for more than six/seven minutes.
Whatever its limitations, Aenima opened new horizons for the entire hard-rock genre.

A Perfect Circle is the project of sound technician Billy Howerdel and Tool's singer Maynard James Keenan, who teamed up with guitarist Troy Van Leeuwen (ex Failure) and a seasoned rhythm section to record an album of avantgarde grunge, Mer De Noms (Sea of Words, 2000). Occasionally super-heavy (Thomas) and catchy (Thinking of You, Judith), but most often bordering on prog-rock (Magdalena), the album incorporates Nine Inch Nails and post-rock in the format of heavy-metal. The strength of the band is particularly visible in songs such as 3 Libras that are both gentle and experimental and songs such as Rose that are both brutal and experimental. The level of pathos is frequently intense (Over, Renholder, Orestes, Brena).

Thirteenth Step (Virgin, 2003) backs up Howerdel and Maynard with former Smashing Pumpkin's guitarist James Iha, drummer Josh Freese, Marilyn Manson's bassist Jeordie White (Twiggy Ramirez). The music of The Package and The Noose is heavy metal drenched in the claustrophobic feeling of early Killing Joke and of Steve Albini's groups. The neurotic tension of The Noose, Blue and Lullaby (Jarboe on vocals) is balanced by the relative linearity of Gravity and Weak and Powerless. Remnants of Tool surface only in The Outsider and Pet. Alas, the pop ballad The Nurse Who Loved Me, written by professional songwriters, not by the band, betrays the commercial ambitions that keep the project from fulfilling its ambitious promises.

A Perfect Circle's eMOTIVe (2004) is an album of cover songs.

After two years of legal fights, Tool returned with Lateralus (Volcano Entertainment, 2001), which stood as both their most experimental and their most visceral album: convoluted counterpoint, ethnic nuances, gothic atmospheres, stately solos. More importantly, Tool had finally mastered the art of the lengthy suite.
Guitarist Adam Jones, vocalist Maynard James Keenan, bassist Justin Chancellor and, best of all, drummer Danny Carey, one of modern rock's true virtuosi refined the album's songs to manic levels.
Most tracks are actually multi-part suites, each movement preparing and defining the next one, in the tradition of a praxis made popular by 1970s' progressive-rock (King Crimson, Van Der Graaf Generator). Thus, the dimensions are monumental, epic, apocalyptic. The eight minute The Grudge (tribal beat, vocal metamorphoses, burning blues guitar), the eight-minute Ticks and Leeches (frantic neurosis, expressionist shouting, quiet strumming, stormy reprise) and the nine-minute Lateralus (martial crescendo, drums and vocals dueting alone, cathartic guitar solo, hushed meditation, raga-like sidereal guitar solo, noisy tense ending) are mini-operas that search the bottom of the soul for active volcanos of passion.
Some of these songs are Freudian sessions that elicit all possible interior demons: Parabol/ Parabola (an Indian-psychedelic lament followed by the album's most virulent riff), The Patient (that starts slow and then soars hymn-like), Schism (Middle-Eastern melody, crashing instrumental crescendos), and finally the almost sacred triptych of Disposition (a subdued meditation with tabla), Reflection (eleven minutes of Middle-Eastern and Indian dances) and Triad (a supersonic and haunting post-Hendrixian instrumental), each of the three songs digging an emotional black hole for the next one to sink into.
This is an imposing, awe-inspiring achievement.

After a five-year hiatus, Tool's fourth album, 10,000 Days (Volcano, 2006), came as a massive disappointment, almost as if Tool had turned into a A Perfect Circle side-project. Wings For Marie, dedicated to Keenan's late mother, and 10,000 Days, with sitar and tabla (and another lyrical solo), which together make up the emotional core of the album, have none of Tool's proverbial power and fire. Vicarious and Right in Two are entertaining and competent and elegant, but certainly nothing that stands out in the age of nu-metal. Countless riffs and melodies and arrangements sounded like imitations of Tool classics.

Keenan also had his own project, Puscifer, that released "V" Is for Vagina (2007) and Conditions of My Parole (2011).

After a hiatus of 13 years, Tool released its fifth album, Fear Inoculum (2019), an 80-minute juggernaut that contains six lengthy pieces and a brief instrumental. The appeal of the ten-minute Fear Inoculum lasts less than three minutes: Danny Carey's tablas and Adam Jones' doom-y oscillating guitar rumble open for a syncopated funk rhythm and Keenan's litany, but then the band doesn't seem to know where to go. The eleven-minute Pneuma begins again with percussion and guitar chiseling an exotic ambient atmosphere but then, again, the times goes by without much happening (the instrumental break for tabla and synth is an amateurish interlude). The 13-minute Invincible opens with the sounds of storm and ocean, and then turns into a melodic power-ballad with atmospheric synth lines; certainly classy pop-metal, but not exactly revolutionary. The 15-minute 7empest begins and ends with a clock-like ticking of guitar and percussion. In between it chugs along with Black Sabbath-ian ferocity, but, other than Jones' guitar workout, it quickly becomes uninteresting. Keenan's sermons are luckily left in the background, while Carey remains a force of nature for the whole 80 minutes, and Jones turns every riff into an epic project. But this album is mostly filled with a lot of dejavu.

A Perfect Circle's aMOTION (Virgin, 2004) contains remixes, and their Stone And Echo (2013) documents a live performance. They returned in earnest with another awful album, Eat The Elephant (BMG, 2018).