Midfielder Ross Barkley has described his £15m move from Everton to Chelsea as "unbelievable" and "a fresh start".
The England international, 24, has signed a five-and-a-half-year deal with last season's Premier League champions.
Barkley, who leaves Everton after 13 years at Goodison Park, has not played for seven months because of a hamstring injury and had surgery in August.
"He's very young and has a lot of space for improvement," said Chelsea manager Antonio Conte.
"He's a good prospect but at the same time, it will be very important to understand, if the player signs, we must have patience to help him recover."
Barkley turned down a £30m move to Stamford Bridge on transfer deadline day in August after rejecting a new deal at Everton.
But with his Goodison contract expiring in the summer, he has now moved for £15m less.
He will wear the number eight shirt at Chelsea - previously worn by Frank Lampard and Oscar.
"To be given a fresh start at a new club like Chelsea, it's unbelievable for me," said Barkley, who made his Toffees debut in 2011.
Barkley joined Everton's academy as an 11-year-old in 2005 and played 150 times in the Premier League, scoring 21 goals.
He has 22 caps for England and played at the 2014 World Cup.
BBC chief football writer Phil McNulty
Barkley's departure brought a low-key ending to a Goodison Park career that always had such high hopes but ends in disappointment and even a tinge of ill-feeling from many supporters - particularly as Everton have lost a large chunk of a potential £30m fee they could have expected had he not turned down a move in August.
Barkley now faces a crucial phase when potential must finally be fulfilled. Previous Everton managers have failed to bring consistency to his game, with Roberto Martinez trying the arm around the shoulder and Ronald Koeman the big stick. Now it is Antonio Conte's turn.
Find all the latest football transfers on our dedicated page.