Everton have signed Chelsea striker Romelu Lukaku for a club record £28m.
The 21-year-old Belgian World Cup forward, who scored 16 goals on loan at Everton last season, has signed a five-year contract at Goodison Park.
Everton boss Roberto Martinez said: "This signing is not just important for this season. It is a significant day in the history of this football club."
Lukaku joined Chelsea from Anderlecht for £18m in August 2011 but played only 15 games for the club.
After signing his contract at Goodison, Lukaku said: "I'm 21, I need to be playing in a good team. I needed to be in a place that felt right.
"I decided very quickly I wanted to come back. This is the place I belong."
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Lukaku's transfer breaks the previous record of £15m for Marouane Fellaini when he joined Everton from Standard Liege in 2008.
It was originally thought Everton would pay £23.7m for Lukaku before the club revealed the £28m fee. Chelsea believe the transfer puts them in a strong position to comply with Uefa's Financial Fair Play rules.
Chelsea boss Jose Mourinho, who has now brought in £68m after the sale of Lukaku and defender David Luiz, said: "He wanted to play for Chelsea but wanted to be the first-choice striker. That's very difficult to promise.
"Everton made an important offer and financial fair play is something that is always behind the thoughts of the board. This leaves Chelsea in a great position."
Martinez said: "We know that Romelu is still a young man and the potential that he has is quite unique, and we are desperate to see him enjoying his football and to watch him grow as footballer in the years to come."
Lukaku rose to prominence as a 16-year-old with Anderlecht in the 2009-10 season when he scored 15 goals to help clinch the Belgium league title.
He scored 16 the next season and 2011-12 made the move to Chelsea, where he played a bit-part role under Andre Villas-Boas and then Roberto Di Matteo.
Lukaku joined West Brom on loan for the 2012-13 season, scoring 17 goals in 38 games as the Baggies recorded their highest ever Premier League finish of eighth.
Then last season, he was top scorer at Everton as they finished fifth in the Premier League and qualified for Europe for the first time in five years.
Lukaku's exploits at Goodison also helped secure a place in Marc Wilmots's Belgium squad for the 2014 World Cup.
He made four appearances in Brazil, scoring one goal, as Belgium reached the quarter-finals, losing 1-0 to Argentina.
Chelsea manager Jose Mourinho had already signed two strikers and let two go this summer.