Keane himself:
"Having received medical advice from my surgeon and the Celtic club doctor, I feel my only option is to retire.
"I would like to send the manager, the staff, players and supporters my very best wishes for the future."
Fergie:
"Roy's obsession with winning and the demands he put on others made him the most influential player in the dressing room.
"He became a great captain through that and, to my mind, he is the best player I have had in all of my time here. Over the years when they start picking the best teams of all time, he will be in there.
"His display in Turin in 1999 was selfless, just wonderful. It was a tragedy that he wasn't able to play in the final in Barcelona."
Pallister:
"I think he is the closest thing to Bryan Robson I've ever seen, you would probably have to put them on a par.
"They were both terrific players for United and if you want to go into a battle they're the type of people you want alongside you.
"He made the game seem so easy, his passing, the energy levels he had and even after his cruciate injury he still inspired his team and his team-mates.
"He amazed everybody. He was a fantastic player, a leader, an inspiration. When things were going tough he was the one the players and the fans looked to to spark things into life.
"None more so than at Juventus in the European Cup when he scored that header that inspired them on to win the trophy. Of course he missed out on the final itself and that is probably one of his biggest regrets but the fans and the players alike will remember him for the player he was on the pitch."
Paul Parker:
"He was one of the best captains Manchester United have ever had, alongside, or maybe ahead, of Bryan Robson.
"When he came to United he was loud and said what he thought - he didn't care what he said to anybody but when you could play the way he could, the players put up with the other side to him."
Gordon Strap-on:
"Roy Keane is one of the greatest ever players to grace the game of football. It was fantastic we were able to bring him to Celtic and it has been a privilege to work with him.
"While we would have very much liked Roy to continue for the remaining year of his contract, everyone at the club fully understands and respects the decision he has made."
Celtic's chief executive Peter Lawwell:
"While we are disappointed, it is important Roy's health comes first and we fully respect the decision which he has made. Despite losing Roy, our supporters can rest assured that we are working hard to strengthen the squad for next season to face the domestic and Champions League challenges which lie ahead."
The Irish taoiseach Bertie Ahern:
"Today is a sad day for his legion of Irish fans, his immense footballing genius is unlikely to ever be forgotten.
"Roy Keane is one of the most decorated Irish players of all time. Over the past decade, he has been arguably the greatest midfield player in world football and has established a reputation as one of the true legends of world football."
The Guardian's David Lacey:
A footballer's true worth is measured in part by the difficulty with which he is replaced and for this reason alone Roy Keane will always be numbered among Manchester United's greatest players. His retirement from the game, following a brief appendix to his career at Celtic, finds United the poorer for his absence.
While recurring injury problems indicated that Keane would have to give up playing sooner rather than later, Sir Alex Ferguson's failed attempts to find the Irishman's successor in midfield have underlined the immense contribution Keane made to Manchester United's growing domination of the English game through the 1990s and beyond.
On the pitch he was Ferguson's alter ego, bringing to the team all the rage for perfection and impatience that the manager demanded. Keane's 12 years at Old Trafford ended abruptly last November following a row with Ferguson over critical comments about the team the United captain had made on the club's television channel, the interview being pulled by MUTV for being too inflammatory.
He had never been one to hold back where what he considered sub-standard performances were concerned but this was clearly a rave too far. In fact Keane had already announced his intention to leave Old Trafford this summer, having been sidelined by a foot injury at Liverpool last September suffered in a tackle with Steven Gerrard.
At least the storminess of Keane's departure was in keeping with much of his playing career. He could enjoy a perfectly clean and disciplined game yet remain a brooding, intimidating presence on the field. Channelled properly this gave United's football an invaluable influence, although when Keane flipped the outcome for an opponent could be painful and nasty.
Ferguson was aware of this. Recalling in his autobiography the incident during the 1995 FA Cup semi-final against Crystal Palace at Villa Park, when Keane was sent off after stamping on Gareth Southgate, the United manager observed: "His Irish fire was fundamental to his immense value as a footballer but his tendency to go beyond the bounds of acceptability would have to be curbed."
Generally Keane was able to preserve a proper balance between what was and what was not acceptable. But the vengeful tackle on Alf Inge Haaland in a Manchester derby and the subsequent admission in his autobiography that the foul was premeditated, which brought him a five-match ban and a record £150,000 fine, left a lingering sour taste.
Eamon Dunphy - Keane's biographer:
"He will be remembered as a great footballer and a very charismatic, candid and intelligent man - quite a unique figure.
"He has taken his coaching qualifications and he would be a great coach and manager. He has all the qualities for the job. He was Sir Alex Ferguson's enforcer on the field at Manchester United, his leader.
"He had turbulent periods in his life. He's an extremely candid man and that can land you in bother from time to time.
"But when the smoke of the controversies fade away, a great career of a great Irish footballer will be remembered."