Casillas began his career in Real Madrid's youth system, known as La Fábrica, during the 1990–91 season. On 27 November 1997, at age 16 and still a junior, he was first called up to the senior team squad to face Rosenborg in the UEFA Champions League, although he remained on the bench throughout. After spending a season with the C-team in the fourth tier, during which they won their regional group, he came into contention as the club's first-choice between the posts. In starting the 1999–2000 UEFA Champions League group stage fixture against Olympiakos on 15 September 1999, he became the youngest goalkeeper ever to feature in the competition at the time, aged 18 years and 177 days; a record which was only broken in October 2017 by Mile Svilar. Three days earlier, Casillas had made his La Liga debut in a 2–2 draw against Athletic Bilbao at San Mamés Stadium. In May 2000, he became the youngest ever goalkeeper to play in and win a Champions League final when Real Madrid defeated Valencia 3–0, just four days after his 19th birthday.