Dream Theater is an American progressive metal band formed in 1985 under the name Majesty by John Petrucci, John Myung and Mike Portnoy — all natives of Long Island, New York — while they attended Berklee College of Music in Boston, Massachusetts. They subsequently dropped out of their studies to concentrate further on the band that would eventually become Dream Theater. Their current lineup consists of Petrucci, Myung, vocalist James LaBrie, keyboardist Jordan Rudess and drummer Mike Mangini.

Over the course of various lineup changes, Petrucci and Myung have been the only two constant members. Portnoy remained with the band until 2010, when he was replaced by Mangini after deciding to leave to pursue other musical endeavors. After a brief stint with Chris Collins, followed by Charlie Dominici (who was dismissed from Dream Theater not long after the release of their first album), LaBrie was hired as the band's singer in 1991. Dream Theater's first keyboardist, Kevin Moore, left the band after three albums and was replaced by Derek Sherinian in 1995 after a period of touring. The band released one album with Sherinian, who was replaced by current keyboardist Jordan Rudess in 1999.

To date, Dream Theater has released fifteen studio albums. The band's highest-selling release is their second album Images and Words (1992), which reached No. 61 on the Billboard 200 chart. The albums Awake (1994) and Six Degrees of Inner Turbulence (2002) also entered the charts at No. 32 and No. 46 respectively, and each received critical acclaim. Their fifth album, Metropolis Pt. 2: Scenes from a Memory (1999), was ranked number 95 on the October 2006 issue of Guitar World magazine's list of the greatest 100 guitar albums of all time. It was also ranked as the 15th Greatest Concept Album in March 2003 by Classic Rock Magazine.As of 2018, Dream Theater has sold over 12 million records worldwide and has received three Grammy Award nominations (including one win in 2022). Along with Queensrÿche and Fates Warning, the band has been referred to as one of the "big three" of the progressive metal genre, responsible for its development and popularization.