The UEFA Champions League first introduced a 16-team round of 16 knockout stage in 2003, during the 2003–04 season. It replaced the competition’s second group stage.
Before the change, the 2002–03 tournament used an initial group phase followed by a second group phase involving 16 teams. The top two clubs from each of those four groups then advanced directly to the quarter-finals. UEFA judged that structure too long and replaced it with a single group stage followed by a first knockout round.
In the new 2003–04 format, 32 clubs were divided into eight groups of four. The top two from each group advanced to the round of 16, where group winners faced runners-up from different groups over two legs. Porto won that inaugural edition of the new format, beating Monaco 3–0 in the final. The answer is sometimes written as 2003–04 because the season began in 2003 and the knockout ties were played in early 2004.