Aged 19, Courtois signed a five-year contract with Chelsea. At his previous club, Racing Genk, he was part of the team that won the 2010/11 Belgian League, the club's first championship in nine years. It was the young keeper's first season as a regular. The year before the club had finished 11th.
The young goalkeeper's contribution earned him the accolades of Belgian Goalkeeper of the Year and Genk's Player of the Year. The team conceded 32 goals over the course of 40 league matches.
At the start of that season he played against Andre Villas-Boas's Porto side, the tie a qualifier for the Europa League which the Portuguese side went on to win.
Courtois is 6ft 6in in height and an Under 18 international. He has been named in the full Belgian national squad but has yet to play.
He first joined Genk at the age of eight and originally played at left-back, switching to between the posts when in the 10/11 age group. He progressed through the youth ranks and made his league debut in April 2009 when aged just 16. He had moved into the squad during the winter break when two keepers departed Genk.
At the start of the 2010/11 season, Genk's number one goalkeeper had problems with his work licence so Courtois started and when he did well, he kept his place, showing calmness under pressure that belied his years.
The league championship was decided in the final play-off game, against Standard Liege who needed to win to be champions. Genk required a draw to win it. With the score 1-1, Courtois made three good saves in the last 10 minutes.