At club level, Casillas started his career with the Real Madrid youth team, eventually gaining promotion to the main team in 1999, where he became the youngest goalkeeper to play in the final of the Champions League, and to win the title, at 19 years and four days. Casillas became the first choice goalkeeper at Real Madrid, winning two Champions Leagues and La Liga titles in his first three seasons, establishing himself as one of the best goalkeepers in the world. During his highly successful career in Madrid, Casillas won all major club titles, including five La Liga titles, four Supercopa de España titles, two Copa del Rey titles, three UEFA Champions Leagues, two UEFA Super Cups, one Intercontinental Cup and the FIFA Club World Cup. After 25 years with Real Madrid, Casillas joined FC Porto in 2015 on a free transfer, where he won the Primeira Liga and a Supertaça Cândido de Oliveira title. While at Porto, he broke the record for most consecutive seasons played in the UEFA Champions League (20) and amassed the most appearances in the competition (177). He ended his playing career in February 2020, after almost a year on the sidelines following a heart attack he suffered during a training session with Porto in May 2019.