Dream Theater is an American progressive metal band formed in 1985 under the name Majesty by John Petrucci, John Myung, and Mike Portnoy while they attended Berklee College of Music in Massachusetts, before they dropped out to support the band. Though a number of lineup changes followed, the three original members remain today along with James LaBrie and Jordan Rudess.

Dream Theater has become a successful progressive metal band. Although the band has had one successful hit ("Pull Me Under" in 1992, which received extensive MTV rotation), they have remained relatively low in mainstream music sales.

The band is well known for the technical proficiency of its instrumentalists, who have won many awards from music instruction magazines. Dream Theater's members have collaborated with many other notable musicians. Guitarist John Petrucci has been named as the third player on the G3 tour six times, more than any other invited guitarist, following in the footsteps of Eric Johnson and Robert Fripp. Drummer Mike Portnoy has won 23 awards from Modern Drummer Magazine and is also the second youngest person (at the age of 37) to be inducted into the Rock Drummer Hall of Fame.

The band's highest selling album is the gold selling Images and Words (1992), which reached #61 on the Billboard 200 charts. Both the 1994 release Awake and their 2002 release Six Degrees of Inner Turbulence also entered the charts at #32 and #46 respectively and received mostly positive reviews. Six Degrees of Inner Turbulence also led to Dream Theater becoming the initial band reviewed in the Music Section of Entertainment Weekly during its opening week of release, despite the magazine generally preferring more mainstream music. In 2007, Systematic Chaos entered US Billboard 200 at #19. Dream Theater has sold over 2.1 million albums in the U.S., and over 8 million records worldwide. The band's tenth studio album, Black Clouds & Silver Linings, was released on June 23, 2009. It entered the US Billboard 200 at #6 and Eurochart Hot 100 at #1, marking their highest entry on either chart.

Dream Theater was formed in Massachusetts in 1985 when guitarist John Petrucci, bassist John Myung, and drummer Mike Portnoy decided to form a band in their spare time while studying at the Berklee College of Music. The trio started by covering Rush and Iron Maiden songs in the rehearsal rooms at Berklee.

Myung, Petrucci, and Portnoy joined together on the name Majesty for their newly formed group. According to the The Score So Far... documentary, they were waiting in line for tickets to a Rush concert at the Berklee Performance Center while listening to the band on a boom box. Portnoy commented that the ending of the song "Bastille Day" (from the album Caress of Steel) sounded "majestic". It was then decided that Majesty would be the band's name.

The trio then set out to fill the remaining positions in the group. Petrucci asked his high school band-mate Kevin Moore to play keyboards. After he accepted the position, another friend from home, Chris Collins, was recruited as lead vocalist after band members heard him sing a cover of "Queen of the Reich" by Queensrÿche. During this time, Portnoy, Petrucci, and Myung's hectic schedules forced them to abandon their studies to concentrate on their music, as they did not feel they could learn more in college. Moore also left his college, SUNY Fredonia, to concentrate on the band.

The beginning months of 1986 were filled with various concert dates in and around the New York City area. During this time, the band recorded a collection of demos, titled The Majesty Demos. The initial run of 1,000 sold out within six months, and dubbed copies of the cassette became popular within the progressive metal scene. The Majesty Demos  are still available in their original tape format today, despite being released officially on CD, through Mike Portnoy's YtseJam Records.

In November 1986, after a few months of writing and performing together, Chris Collins was fired. After a year of trying to find a replacement, Charlie Dominici, who was far older and more experienced than anyone else in the band, successfully auditioned for the group. With the stability that Dominici's appointment brought to Majesty, they began to increase the number of shows played in the New York City area, gaining a considerable amount of exposure.

Shortly after hiring Dominici, a Las Vegas group also named Majesty threatened legal action for intellectual property infringement related to the use of their name, so the band was forced to adopt a new moniker. Various possibilities were proposed and tested, among them Glasser, Magus, and M1, which were all rejected, though the band did go as Glasser for about a week, though fans reacted poorly to this decision. it wasn't until Portnoy's father suggested the name Dream Theater, the name of a movie house in Monterey, California.

With their new name and band stability, Dream Theater concentrated on writing more material while playing more concerts in New York and in neighboring states. This eventually attracted the attention of Mechanic Records, a division of MCA. Dream Theater signed their first record contract with Mechanic on June 23, 1988 and set out to record their debut album. The band recorded the album at Kajem Victory Studios in Gladwyne, Pennsylvania. Recording the basic tracks took about 10 days, and the entire album was completed in about 3 weeks.

When Dream and Day Unite was released in 1989 to far less fanfare than the band had anticipated. Mechanic ended up breaking the majority of the financial promises they had made to Dream Theater prior to signing their contract, so the band was restricted to playing around New York City. The promotional tour for the album consisted of just five concerts, all of which were relatively local. Their first show was at Sundance in Bay Shore, New York opening for the classic rock power trio Zebra.

After the fourth show, Dominici was fired because of personal and creative differences. Shortly after, however, the band Marillion asked Dream Theater to open for them at a gig at the Ritz in New York, so Dominici was given the opportunity to perform one last time. It would be another two years before Dream Theater had a replacement vocalist.

The members of Dream Theater have many times stated that their music was inspired by various musicians such as Asia, Boston, Dio, Emerson, Lake & Palmer, Iron Maiden, Judas Priest, Kansas, King Crimson, Led Zeppelin, Yngwie Malmsteen, Marillion, Megadeth, Steve Morse, Ozzy Osbourne, Rainbow, Styx, Triumph, Yes, GTR, Tempest, Pink Floyd, Genesis, Metallica, Queen, Queensrÿche, Rush, Black Sabbath, Deep Purple, Jethro Tull, Frank Zappa, and U.K..