Chicago (retroactively known as Chicago II) is the second studio album by Chicago-based American rock band Chicago. Like their debut album, Chicago Transit Authority, this was a double album. It was their first album under the name Chicago (the band's prior name, Chicago Transit Authority, was changed due to a threatened lawsuit from the actual mass-transit operator bearing the same name) and the first to use the now ubiquitous cursive Chicago logo on the cover. Released in January 1970 on Columbia Records, Chicago was commercially successful. It was certified gold by the Recording Industry Association of America (RIAA) in April of the same year of its release, and certified platinum in 1991. It reached No. 4 on the album charts in the United States and No. 6 on the album charts in the UK, and produced three top ten singles on the Billboard Hot 100. The album received three Grammy Award nominations - for Album of the Year, Contemporary Vocal Group, and Best Album Cover. It was voted best album of 1970 by readers of Cash Box magazine, and the 1971 best small-combo LP by readers of Playboy magazine.

Chicago was founded in 1967 by Walt Parazaider (woodwinds), James Pankow (trombone), Lee Loughnane (trumpet), Roert Lamm (keyboards and lead vocals), Terry Kath (guitar and lead vocals), and Danny Seraphine (drums). They called themselves The Big Thing. They added bassist-lead vocalist Peter Cetera and began working with their original producer-manager James William Guercio, subsequently becoming known as Chicago Transit Authority. Their self-titled 1969 debut release was an ambitious mix of free-form rock and horn-influenced jazz, the first of three double LPs that would start their career. The band dropped the "Transit Authority" and became known as simply "Chicago" on their second album in 1970, which was the first album to carry their trademark logo. CHICAGO II yielded some of the group's most enduring hits, which came from Pankow's suite "Ballet For A Girl In Buchannon," emulating the form of the great classical composers from a pop perspective. The album peaked at #4 on the Billboard charts. With the success of CHICAGO II, radio went back and discovered hits on the earlier CHICAGO TRANSIT AUTHORITY album. Their 1971 release CHICAGO III peaked at #2, driven by the success of the tracks "Free" and "Lowdown."