The knockout stage of the 1998–99 UEFA Champions League began on 3 March 1999, and ended with the final at the Camp Nou in Barcelona, Spain, on 26 May 1999. The six group winners in the group stage, as well as the two best runners-up, competed in the knockout stage. For the quarter-finals, two group winners were randomly drawn against the two best runners-up from another group while the other four group winners face each other with the restriction that the two best runners-up cannot be drawn against the winners of their own group. The four quarter-final winners were then drawn together for the semi-finals, the winners of which contested the final.
Each quarter-final and semi-final was played over two legs, with each team playing one leg at home; the team that scored the most goals over the two legs qualified for the following round. If the two teams scored the same number of goals over the two legs, the team that scored more goals away from home qualified for the next round; if both teams scored the same number of away goals, matches would go to golden goalextra time and then penalties if the teams could not be separated after extra time.
Note: In 1991–92, the competition was still known as the European Cup, but is included as it was the first to use a group stage format. In that season and 1992–93, there was no knockout phase between the group stage and final.
