The knockout stage of the 2003–04 UEFA Champions League ran from 24 February 2004 until the final at the Arena AufSchalke in Gelsenkirchen, Germany, on 26 May 2004.
The team that had the higher aggregate score over the two legs progressed to the next round.
If away goals were also equal, 30 minutes of silver goal extra time was applied, whereby the team who leads the game at the half-time break during the extra time period would be declared the winner.
If no goals were scored during extra time, the tie was decided via a penalty shoot-out.
The draw mechanisms for each round is as follows: In the final, the tie was played over just one leg at a neutral venue.
The final was played on 26 May 2004 at the Arena AufSchalke in Gelsenkirchen, Germany.