They are some of the best players in the world and are only one game from getting their hands on the Champions League trophy.
But who are the key players who will make the difference at Wembley? Newsround brings you two five-a-side teams of the players each team couldn't do without.
MANCHESTER UNITED: EDWIN VAN DER SAR. He might be 40, and the final his last match before he retires - but the big Dutch man is still one of the best in the business
NEMANJA VIDIC. Fast, strong and as hard as nails - Vidic is one of the best defenders around. He'll have to be at his best if he's going to stop Barcelona though.
PARK JI-SUNG. Gets more fan mail than any other player at United - but he's not one of their best-known players. Sir Alex rates him in European games - he never stops running!
BAD BOY: PAUL SCHOLES. With amazing shooting and passing, Scholes has been key for years. But tackling has never been his strength and he's been sent off in the past.
STAR MAN: WAYNE ROONEY. United would be lost without Wazza. Has to be at his best if United are to beat the best team in the world.
BARCELONA: DANI ALVES. He's a defender - sort of. Expect the Brazilian right to bomb up and down the wing and get involved in the attacking - whipping in some devilish crosses.
XAVI. Man of the match last time these two played in the final in 2009. The heartbeat of the team and has won everything - expect him to pass, pass and pass some more.
ANDRES INIESTA. Like Xavi he loves to pass - but can dribble and has lots of tricks. Won the World Cup scoring the winning goall, and Rooney thinks he's the best in the business.
BAD BOY: CARLES PUYOL. Don't let his long locks fool you - he's not to be messed with. Just loves arguing and tackling - will let Wayne Rooney know he's got a game on his hands.
STAR MAN: LIONEL MESSI. What else is there to say? The best player in the world - and maybe one of the best ever! If he's in the mood no-one can stop him.