The 21-year-old has enjoyed two successful campaigns in the Spanish capital and has played an important part in Diego Simeone's first-team squad.
With Petr Cech still holding on to the number one spot at Stamford Bridge, Courtois is not guaranteed regular action at Chelsea, so the Europa League champions have allowed him to continue his development elsewhere.
However, if Atletico Madrid are serious about taking the Belgium international to the Vicente Calderon for a third successive year, then they will have to pay the price.
Ross Turnbull and Hilario are both out of contract this summer, and so if Courtois leaves west London again, Chelsea will be forced to either offer one of their current back-ups a new deal, or alternatively sign a new understudy for Cech for 2013/14.
There's no doubt that Courtois is seen as the long-term successor to Cech, so Chelsea will be keen to keep hold of a player that is yet to make an appearance for the club since signing from Racing Genk in 2011.