Aston Villa have completed the signing of Watford forward Ashley Young for an initial fee of £8m.
The 21-year-old has signed a four-and-a-half-year deal at Villa Park, with clauses meaning his price tag could end up reaching £9.65m.
"He has got enormous potential and I think he will fulfil that," said Aston Villa manager Martin O'Neill.
"Ashley is a really talented player. I have seen a lot of him and I am going to back my judgement."
The England Under-21 international, who scored 22 goals in 83 starts for Watford, turned down a move to West Ham earlier this month.
He told the Villa website: "This is an exciting opportunity and I can't wait to get started.
"I'm glad it's all done and dusted with the medical and I can't wait to train.
"There's a good set of boys here and I just want to get down to work now. We've got a break next weekend, which is disappointing.
"I believe he has the ability to go on and achieve things"
"But I can get to know the lads and prepare for a debut at Newcastle next week - that's not a bad stage to make my debut."
Young's basic deal is believed to be worth about £30,000 a week, and he is expected to make his Villa debut against Newcastle on 31 January.
"I had been hopeful Ashley would come to Aston Villa after he verbally agreed to join us over the weekend but you never take anything for granted until they sign on the dotted line," added O'Neill.
"You think along the lines of `Will he be of value over the next four or five years' and you base your decision on that.
"He can play on both flanks and he can play alongside the centre-forward.
"I think he is an exciting player and has got a great touch, great control, plus a wee bit of pace. I am excited with his signing for us.
"He is only a young fellow and it will be hard to put him under the sort of pressure where he is having to justifying himself from minute one, but overall I have no doubt at all he is good enough.
"I know his potential but it is different knowing the potential of a player and what they eventually go on to achieve. I believe he has the ability to go on and achieve things."
Watford boss Aidy Boothroyd said as many as seven Premiership clubs had expressed an interest in signing Young.
And Boothroyd backed his former player to take his game to the next level under O'Neill.
"Ashley is worth the transfer fee - he might even be a bargain"
"I think Ashley can kick on again because of his mental capabilities and the manager of the club he's going to," he said.
"I have a great deal of time and a great deal of respect for Martin O'Neill and I think anybody who goes to the club he is in charge of is very, very lucky.
"I think Ashley is worth the transfer fee. He might even be a bargain.
"He can cope with the expectations ahead. He is a very strong kid, very disciplined, the ultimate professional and he is going to have a very good career."
Young broke into Watford's first team in 2003 and established himself as a talent to watch last season, with 15 goals in the Hornets' successful push for promotion.
He becomes the club's second signing of the week following the arrival of John Carew from Lyon in a swap deal with Milan Baros.