Cesc Fabregas was one of the stars of the Premier League during his eight years with London rivals Arsenal before he rejoined Barcelona, his boyhood club, in 2011.
His return to London looks like it could be one of the most significant signings in English football this summer.
Chelsea coach Jose Mourinho hopes Fabregas will provide the creative spark from midfield that contributed to Chelsea's struggle to score goals in the 2013/14 season.
Mourinho's side won all but one of their encounters with the other teams that finished in the top four last season. But they often failed to break down the massed defences of less ambitious sides, dropping the points which cost them the title.
The Portuguese coach sees Fabregas fitting the hole left by Frank Lampard whose attacking instincts made him Chelsea's all-time top scorer despite playing in midfield. Lampard left Chelsea at the end of last season after 13 years with the club.
"I see him everywhere, because he can play everywhere," Mourinho said of Fabregas. "His best quality is the speed with which he analyses the game and executes it. He's very intelligent. We need his football brain in our team."
Fabregas will be expected to help bring out the best in Chelsea's other big signings, including Diego Costa who was bought from Atletico Madrid in response to the continued inconsistency of fellow Spain striker Fernando Torres.
Costa scored 64 goals in 134 appearances for Atletico, a stark contrast with Torres at Chelsea who found the net only 11 times last season.
Torres may even be edged out further by the return to London of Didier Drogba, now 36 but guaranteed a hero's welcome by Chelsea's fans. They will never forget the Ivorian's last appearance for the Blues in the Champions League final of 2012 when he played a huge part in the win over Bayern Munich.
With Mourinho under pressure to win silverware after a barren first season back in London, his priority has been to find a way to get Chelsea to score more goals.
But he has also shored up the Premier League's meanest defence from last season - Chelsea conceded 27 goals in 38 games, 10 fewer than champions Manchester City.
Brazilian left back Filipe Luis and Belgian goalkeeper Thibaut Courtois both arrived from Atletico Madrid after impressing in the Spanish club's elimination of Chelsea in the semi-finals of last season's Champions League.
Courtois, loaned by Chelsea to Atletico for the past three seasons, will battle for a starting place against veteran Petr Cech who, at 32, remains one of the best keepers in the world.
"If I have to make a decision for Cech to be on the bench, or for Courtois to be on the bench, it's no problem for me," Mourinho said. "I will do what I think is the best."