"Karn Evil 9" is an extended work by progressive rock group Emerson, Lake & Palmer, appearing on the album Brain Salad Surgery. A futuristic fusion of rock and classical themes, it is regarded by some to be their best work together with the song "Tarkus". At nearly half an hour long, it is also their longest studio recording.

"Karn Evil 9" was written by Keith Emerson, Greg Lake, and former King Crimson lyricist Peter Sinfield. The phrase "Karn Evil" is an alteration of the word "carnival".

It is the fifth and final track on Brain Salad Surgery with a running length of 29 minutes and 37 seconds. The initial release of the album on vinyl found "Karn Evil 9" split between the two sides due to its length, with a fade out/fade in between First Impression parts 1 and 2. Subsequent releases on compact disc and digital download have "Karn Evil 9" as a single track.

"Karn Evil 9" consists of three movements (called "impressions") with the first impression divided into two parts:

The work's most recognizable portion to many is First Impression Part 2 and its introduction ("Welcome back, my friends / To the show that never ends"), which eventually became the title of their second live album. It is often used as a song dedicated to the opening of sports seasons. First Impression Part 2 is also a common radio staple, although it opens with the intro line and can leave casual radio listeners a bit surprised, not necessarily knowing what led up to that point from Part 1 (the common problem being that combined, First Impression lasts over 13 minutes, and hence would not be well accessible on radio played with Parts 1 and 2 together due to its length, so the shorter Part 2 is just played by itself instead).

The story of "Karn Evil 9" is told in three parts, with the second part being an instrumental interlude. First Impression begins with a tale of a bleak world (although timeframe is not specified): "Cold and misty morning I heard a warning borne on the air ..." and where humanity is either being destroyed or has fallen into decay and helplessness. The decadence of the old world is preserved through exhibits that are part of a futuristic carnival show, which exhibits depravities like "seven virgins and a mule," along with things that are rare in the future, such as a "real blade of grass."

Unlike the rest of "Karn Evil", the Second Impression's instrumentation is primarily piano, bass, and drums with a solo by Emerson on a Minimoog set to imitate a steelpan. There is supposedly a vocal around 2 minutes (15 minutes as a whole song) that sounds like a small child's voice saying "Daddy, let's go see the carnival!" as reference to the first part. However, its is actually Emerson's voice, just sped up and altered. This impression changes from an upbeat out-of-control tune to a slow interval and then picks up the pace with a structure similar to that of a sonata. It is allegedly about computers scheming against the humans, and the humans completely unsuspecting this. This Impression is often overlooked and is less popular than the others,though it is a rather complex piece, showing the three musicians' virtuosity.

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