Thanks for joining us tonight, stay tuned for a full match report and all the best reaction.
Bayern Munich can start booking their tickets for Wembley now - they have comprehensively demolished Barcelona and it will take an astonishing comeback in the second leg to turn this semi-final around.
Bayern take it short and referee Kassai brings the game to an end.
Robben's shot is deflected behind for a corner.
We're into the final few seconds of what has been an astonishing evening in Munich.
Now Iniesta goes into the book for bringing down Martinez.
Every Bayern touch is now being greeted by cheers from the home supporters.
Three additional minutes will be played.
Ridiculous from Jordi Alba as he throws the ball into the face of Robben. He is booked and will miss the second leg.
Bayern make their final change with Ribery making way for Xherdan Shaqiri.
Barcelona are in bits here, with Pique rolling a simple pass straight out of play under pressure from Robben.
And now Schweinsteiger joins him for invoking the wrath of referee Kassai.
Sanchez goes into the book for pulling back Robben.
Barcelona also make a change with David Villa coming on for Pedro.
Muller immediately makes way for Pizarro.
Barcelona are cut open once again as Ribery feeds Alaba down the left with his dangerous low cross turned home at close range by Muller. What a scoreline!
GOAL FOR BAYERN MUNICH!!!! Muller scores again!
Alves is down needing treatment after being caught by Schweinsteiger in the scramble after Valdes' save. He limps to the sidelines.
Great save from Valdes to deny Muller as he breaks in to the Barcelona area.
Awful, awful miss from Bartra. He gets in behind the Bayern defence from Xavi's clipped free-kick but wants far too much time and eventually skews his attempted shot horribly off target.
Barcelona are hit on the break, but there is a huge amount of controversy over the goal, with Muller's blatant body-check on Jordi Alba creating the space for Robben to beat Valdes.
GOAL FOR BAYERN MUNICH!!! Robben makes it 3-0!
Pedro's dangerous whipped cross is pushed away from Pique's head by Neuer.
Bayern make the first change of the night, with goalscorer Mario Gomez replaced by Luiz Gustavo, who will add more bite to midfield.
Another corner from Robben causes Valdes more problems as he leaps into Gomez and gets away with winning a free-kick.
Neuer is called into action as the corner is headed down to Sanchez who stabs a shot straight at the grateful keeper.
Glimpse of the real Messi as he tricks his way into the area before earning a corner.
Barcelona are enjoying possession in far more threatening positions on the edge of the Bayern area, but the home defence is standing firm at the moment. Sanchez fires an attempted cross behind Neuer's goal.
Better spell of play from Barcelona ends when Alves' cross is headed away by Dante.
Heavy challenge from Schweinsteiger on Xavi, but he escapes any further punishment than the free-kick.
Xavi finds Dani Alves bursting down the right with his pull-back forcing Neuer to dive on the ball for once.
Robben bursts forward and has the Barcelona defence on the back foot, but his attempted pass to Ribery is too strong.
Robben gets up well to meet Schweinsteiger's curling free-kick into the box but flashes a header narrowly wide of the post.
Messi drifts out to the right to try and find some space and then cuts inside, but he again loses possession on the edge of the Bayern area.
Bayern go close to a third as Robben cuts inside and then feeds Ribery who drags his shot across the face and wide.
Barcelona are really up against it now and are playing with a rare lack of conviction and verve, with the returning Messi largely anonymous.
Very poor goalkeeping from Valdes on the second Bayern goal, as he failed to come for the corner with any conviction, although there was more than a hint of offside as Gomez finished.
Muller cuts across the Barcelona box and then turns back to fire a shot on goal which flies narrowly wide.
All too easy for Bayern Munich as they double their lead from a corner. Robben's deep kick is headed back across the box by Muller and Gomez is on hand to turn it home from point-blank range.
GOAL FOR BAYERN MUNICH!!! Gomez doubles their lead!!
Great play from Iniesta as he plays a one-two with Pedro and bursts into the box. Neuer, however, gets to the ball first and the flag is also up for offside.
Very early booking in second period for Martinez as he goes into the back of Iniesta.
Barcelona get the second period under way.
The two teams are out and the second half is moments away.
The Bayern groundstaff have been working on the sodden centre of the pitch during the interval, hopefully it will now be a bit less damp.
Referee Kassai brings the half to a close. Barcelona have had more of the ball, but less of the game. Bayern have created the better chances and deserve their interval lead.
Xavi's kick is easily cleared by Gomez before Alves' attempted return pass drifts out of play.
Corner for Barca as half-time approaches.
Messi attempts a pass through the Bayern defence to the on-rushing Jordi Alba, but it is just too strong.
Sanchez is penalised for jumping into Martinez in the Bayern box.
Messi wins a free-kick in his own half after two swift challenges from Gomez and Muller, with Xavi complaining to the referee.
Now Bartra goes into the book for leaping into the back of Muller.
Xavi's neat pass goes towards Alves towards the far post. Ribery attempts to give the ball back to Neuer and only finds the Barca man, but the hosts clear.
First booking of the night as Gomez clatters into Messi. Easy decision for the referee.
Schweinsteiger's delivery is poor and Jordi Alba can clear from the near post.
Good play from Bartra as he wins possession from Ribery, but Bayern go back on the attack and win another corner.
Muller has a chance to shoot but instead elects to attempt to clip a ball over the top to Gomez, but has too much weight on the pass.
Ribery's latest corner is met by Dante's head before striking Sanchez's arm as he challenges the Bayern man, but referee Kassai is again unimpressed.
Messi gets away from Ribery in midfield but Schweinsteiger slides in to deny the Barcelona man.
BARCELONA GO CLOSE!!! Pedro's great ball into the box looks destined for Messi but Dante gets a toe to the ball and manages to divert it away from Neuer's goal.
Dani Alves has the chance to cross but over-hits the ball way beyond the danger area.
Muller looks to release Robben into space behind the Barcelona defence but Pique reads the danger well and clears.
Ribery's initial corner was poor, but Bayern kept possession and Robben's cross from the right was met by Dante's head, with Muller arriving at pace to head home.
GOAL FOR BAYERN MUNICH!!!! Muller finds the net.
Now it's Bayern's turn to have a spell of possession, before Lahm's long ball towards Gomez forces Bartra to concede another corner.
Poor pass from Bartra gives Bayern the ball and they win a corner. Valdes and Busquets are down briefly as Martinez stands his ground and the visitors have a free-kick.
Great skill from Iniesta to beat Lahm out on the left, as Barcelona look to work an opening, but it comes to nothing.
More attacking play from Lahm as he wins a corner after surging in to the Barca box, but the home side waste it with Robben carelessly offside.
Bayern keep the pressure on from the corner and Muller's shot from a tight angle is gathered by Valdes.
Lahm is allowed to advance towards the Barcelona box and he unleashes a shot which hits Pique and goes out for a corner. Bayern penalty appeals are waved away as the ball hit Pique.
They had the sprinklers on the surface here before kick-off and it looks like they may have over-done things in a couple of areas, with the centre-circle particularly damp.
More possession play from Barca, but it's inside their own half and they won't hurt Bayern from there.
Messi attempts to trick his way beyond Ribery on the right flank, but the Bayern man is not buying it and the ball goes out for a throw to the home side.
Pique attempts to spread the play out to Alves on the flank but his radar is off once again.
Xavi's corner goes all the way through to Pique, who gets there ahead of Dante, but cannot keep his header on target.
Alves feeds Pedro on the right of the box and he earns a corner after a challenge from Alaba.
First touch of the night for Messi, with three Bayern players quick to close him down, even on the halfway line near the touchline, with the Barca man neatly finding a team-mate.
Pique heads away and Barcelona complete the clearance as Martinez tumbles under Bartra's challenge, but both men had their eyes fixed on the ball in the air.
Ribery feeds Alaba down the left and he wins a corner from Alves.
Pique attempts a long ball forward but it drifts harmlessly out of play.
CHANCE FOR BAYERN!!! Robben plays a neat one-two with Javi Martinez and gets his shot away but Valdes gets down well to block.
Bayern win the ball and Muller feeds Robben down the right, he plays it back to Lahm, but his cross is headed away.
Barcelona are immediately on the ball and knocking it around among their defenders to give everyone an early touch.
Bayern get the game under way.
Simon Wheatley writes: "This is going to be the game of the season so sit down, relax, have a drink and watch a brilliant game of football and enjoy." Sound advice.
Impressive scene in the Allianz Arena with the home supporters holding aloft red and white banners to make an impressive mosaic all around the ground.
The match officials and the two teams are in the tunnel and heading out towards the playing surface.
Barcelona are no slouches in front of goal, either, with the Catalan club finding the net 14 times in their last six games, three of which they have won and three of which ended in draws, including both legs of their quarter-final against Paris St Germain.
It would be fair to say Bayern Munich have been in decent form recently, with their last six games seeing them score 24 goals while conceding just three. They won all six, in case anyone was wondering...
There are still 15 minutes to kick-off in the Allianz Arena but the atmosphere in the ground is electric.
Our friends at Sky Bet have a host of markets available on tonight's clash, but the main prices are: Bayern Munich on offer at evens, the draw is available at 13/5 with Barcelona currently an 11/4 shot.
Opta stat No.7: Lionel Messi has been directly involved in six of Barcelona's eight away goals in the Champions League this season (four goals, two assists).
Opta stat No.6: Barcelona have completed 7,220 passes, that's 2,555 more than the second placed side in that department, Bayern Munich (4,665).
Opta stat No.5: Barcelona have had 56 shots on target in the Champions League this season, the lowest tally among the four semi-finalists. Bayern Munich have had 66.
Opta stat No.4: Bayern Munich have not kept a clean sheet in their last 12 Champions League games at home against Spanish opposition. However, Bayern have always scored at home in the Champions League against Spanish opposition (13 games), scoring 25 goals in total.
Opta stat No.3: Bayern Munich have won 17 of their last 20 Champions League games played at the Allianz Arena.
Opta stat No.2: Barcelona have reached the Champions League semi-finals for the sixth consecutive season, a record in the history of the competition, while this is the third time in the last four seasons that Bayern Munich have reached the last four of the competition.
Opta stat No.1: Barcelona have only won one of their six competitive encounters with Bayern Munich (two draws, three defeats).
Time for a few stats from our friends at Opta prior to kick-off.
Bayern Munich captain Philipp Lahm and fellow defender Dante are one booking away from a suspension which would rule them out of the second leg, while Barcelona duo Gerard Pique and Jordi Alba are walking the same disciplinary tightrope.
Chris Green from Huddersfield writes: "This game is going to be a cracker and I hope there will be lots of goals for us to enjoy. I will be backing Bayern Munich all the way this evening but I don't think they'll win if they don't perform to their potential. If they are to have a chance of taking a healthy lead back to Spain, they must stop Barcelona in their tracks; otherwise, they will get punished because Barcelona have the players to hurt any team. My prediction for tonight's match: Bayern Munich 3-2 Barcelona."
Simon Bevis is slightly more cynical about Messi's return to action and says: "Hamstring to go after four minutes."
Harry Richardson is the first to get in touch to have his say and he reckons: "I think 2-1 to the Spanish giants! Messi scoring them both and Robben to get Bayern's goal."
Tonight's referee is Viktor Kassai from Hungary.
Barcelona: Valdes; Alves, Bartra, Pique, Alba; Iniesta, Xavi, Busquets; Pedro, Messi, Sanchez. Subs: Pinto, Fabregas, Villa, Thiago Alcantara, Montoya, Abidal, Song.
Bayern Munich: Neuer; Lahm, Dante, Boateng, Alaba; Schweinsteiger, Martinez; Robben, Muller, Ribery; Gomez. Subs: Starke, Van Buyten, Shaqiri, Rafinha, Pizarro, Luiz Gustavo, Tymoshchuk.
A statement from Barcelona on Messi read: "Leo Messi has received the medical all-clear and will be able to play against Bayern Munich in the first leg of the Champions League semi-finals. The Argentine player has the green light from the doctors to play in the huge match against the German side."
If you want to have your say on tonight's match then drop me a line to firstname.lastname@example.org or via Twitter @skysportsrobp and I'll publish a selection of the best prior to kick-off.
We'll bring you the confirmed starting XIs for both teams just as soon as we have them.
Messi is expected to take his place in the starting XI tonight having been forced to step off the bench to rescue his side in the second leg against PSG.
The major news ahead of tonight's game is that Barcelona ace Lionel Messi has been passed fit after suffering a hamstring injury in the first leg of the quarter-final with Paris St Germain.
Good evening and welcome to our live and interactive coverage of tonight's UEFA Champions League semi-final first leg.