Their 1991 album Achtung Baby and the accompanying Zoo TV Tour were a musical and thematic reinvention for the band. Reacting to their own sense of musical stagnation and a late-1980s critical backlash, U2 incorporated dance music and alternative rock influences into their sound and performances, abandoning their earnest image for a more ironic, self-deprecating tone. Similar experimentation continued for the remainder of the 1990s. Since 2000, U2 have pursued a more conventional sound, while maintaining influences from their previous musical explorations.

U2 have released 12 studio albums and are among the most critically and commercially successful groups in popular music. They have won 22 Grammy Awards, more than any other band, and they have sold more than 150 million records. In 2005, the band were inducted into the Rock and Roll Hall of Fame in their first year of eligibility. Rolling Stone magazine ranked U2 at number 22 in its list of the "100 Greatest Artists of All Time". Throughout their career, as a band and as individuals, they have campaigned for human rights and philanthropic causes, including Amnesty International, the ONE/DATA campaigns, Product Red, and The Edge's Music Rising.

The band formed in Dublin on 25 September 1976. Larry Mullen, Jr., then 14 years old, posted a notice on his secondary school (Mount Temple Comprehensive School) notice board in search of musicians for a new band. Setting up in his kitchen, Mullen later described it as "'The Larry Mullen Band' for about ten minutes, then Bono walked in and blew any chance I had of being in charge." Mullen was on drums, Paul Hewson (Bono) on lead vocals, Dave Evans (The Edge) and his older brother Dik Evans on guitar, Adam Clayton, a friend of the Evans brothers on bass guitar, and initially Ivan McCormick and Peter Martin, two other friends of Mullen. Soon after, the group settled on the name "Feedback", because it was one of the few technical terms they knew. Martin did not return after the first practice, and McCormick left the group within a few weeks. Most of the group's material initially consisted of cover versions, which the band said was not their forté. The band's early original material was influenced by punk rock acts, such as The Jam, The Clash, and The Sex Pistols.[citation needed]

"We couldn't believe it. I was completely shocked. We weren't of an age to go out partying as such but I don't think anyone slept that night.... Really, it was just a great affirmation to win that competition, even though I've no idea how good we were or what the competition was really like. But to win at that point was incredibly important for morale and everyone's belief in the whole project."

In March 1977, the band changed their name to "The Hype". Dik Evans, who was older and by this time at college, was becoming the odd man out. The rest of the band was leaning towards the idea of a four-piece ensemble and he was "phased out" in March 1978. During a farewell concert in the Presbyterian Church Hall in Howth, which featured The Hype playing covers, Dik ceremoniously walked offstage. The remaining four band members completed the concert playing original material as "U2". Steve Averill, a punk rock musician and family friend of Clayton's, had suggested six potential names from which the band chose "U2" for its ambiguity and open-ended interpretations, and because it was the name that they disliked the least.

On Saint Patrick's Day in 1978, U2 won a talent show in Limerick, Ireland. The prize consisted of £500 and studio time to record a demo which would be heard by CBS Ireland. This win was an important milestone and affirmation for the fledgling band. The band recorded their first demo tape at Keystone Studios in Dublin, in May 1978. Hot Press magazine was influential in shaping the band's future; in May, Paul McGuinness, who had earlier been introduced to the band by the magazine's journalist Bill Graham, agreed to be U2's manager. U2's first release, an Ireland-only EP entitled Three, was released in September 1979 and was the band's first Irish chart success. In December 1979, U2 performed in London for their first shows outside Ireland, although they failed to get much attention from audiences or critics. In February 1980, their second single "Another Day" was released on the CBS label, but again only for the Irish market.

Island Records signed U2 in March 1980, and in May, the band released "11 O'Clock Tick Tock" as their first international single. The band's debut album, Boy, followed in October. Produced by Steve Lillywhite, it received generally positive reviews. Although Bono's lyrics were unfocused and seemingly improvised, a common theme was the dreams and frustrations of adolescence. The album included the band's first United Kingdom hit single, "I Will Follow". Boy's release was followed by the Boy Tour, U2's first tour of continental Europe and the United States. Despite being unpolished, these early live performances demonstrated U2's potential, as critics noted that Bono was a "charismatic" and "passionate" showman.

The band's second album, October, was released in 1981 and contained overtly spiritual themes. During the album's recording sessions, Bono and The Edge considered quitting the band due to perceived spiritual conflicts. Bono, The Edge, and Mullen had joined a Christian group in Dublin called the "Shalom Fellowship", which led them to question the relationship between the Christian faith and the rock and roll lifestyle. Bono and Edge took time off between tours and decided to leave Shalom in favour of continuing with the band. Recording was further complicated when a briefcase containing lyrics for several working songs was stolen from backstage during the band's performance at a nightclub in Portland, Oregon. The album received mixed reviews and limited radio play. Low sales outside the UK put pressure on their contract with Island and focused the band on improvement.

Resolving their doubts of the October period, U2 released War in 1983. A record where the band "turned pacifism itself into a crusade", War's sincerity and "rugged" guitar was intentionally at odds with the "cooler" synthpop of the time. The album included the politically charged "Sunday Bloody Sunday", where Bono had lyrically tried to contrast the events of Bloody Sunday with Easter Sunday. Rolling Stone magazine wrote that the song showed the band was capable of deep and meaningful songwriting. War was U2's first album to feature the photography of Anton Corbijn, who remains U2's principal photographer and has had a major influence on their vision and public image. U2's first commercial success, War debuted at number one in the UK, and its first single, "New Year's Day", was the band's first hit outside Ireland or the UK.

On the subsequent War Tour, the band performed sold-out concerts in mainland Europe and the US. The sight of Bono waving a white flag during performances of "Sunday Bloody Sunday" became the tour's iconic image. U2 recorded the Under a Blood Red Sky live album on this tour, as well as the Live at Red Rocks concert film, both of which received extensive play on the radio and MTV, expanding the band's audience and showcasing their prowess as a live act. Their record deal with Island Records was coming to an end, and in 1984 the band signed a more lucrative extension. They negotiated the return of their copyrights (so that they owned the rights to their own songs), an increase in their royalty rate, and a general improvement in terms, at the expense of a larger initial payment.