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 knockout stage involved the 16 teams that finished in the top two in each of their groups in the group stage.

Times are CET/CEST,^{[note 1]} as listed by UEFA (local times are in parentheses).

Each tie in the knockout stage, apart from the final, was played over two legs, with each team playing one leg at home. The team that had the higher aggregate score over the two legs progressed to the next round. In the event that aggregate scores finished level, the team that scored more goals away from home over the two legs progressed. 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 the scores were still level after the initial 15 minutes of extra time, play would continue for a further 15 minutes. The away goals rule applied again after extra time. If there were goals scored during extra time and the aggregate score was still level, the visiting team qualified by virtue of more away goals scored. 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 draw for the round of 16, matches were played between the winner of one group and the runner-up of a different group, with the group winner hosting the second leg. Teams from the same country or the same group must not be drawn against each other.
• In the draw for the quarterfinals onwards, these restrictions do not apply, and teams from the same group or same association may be drawn together.

In the final, the tie was played over just one leg at a neutral venue. If scores were level at the end of normal time in the final, then 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 the scores were still level after the initial 15 minutes of extra time, play would continue for a further 15 minutes. If teams still tied, the tie was decided by penalty shoot-out.

As winners of the competition, Porto went on to represent UEFA at the 2004 Intercontinental Cup.

More information Monaco, 0–3 ...

Monaco	0–3	Porto
	Report	Carlos Alberto 39' Deco 71' Alenichev 75'

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• 2003-04 season at UEFA website Archived 2010-01-23 at the Wayback Machine