Barcelona become the first team to overturn a 2-0 first-leg deficit in the Champions League era. They started like a train, got in front on the night, and Milan never really recovered. The Barcelona Crisis is over. Messi the star with the two first-half strikes that brought the scores level but in truth all four of Barca's goals were a little bit special. You wouldn't bet against them lifting the big trophy at Wembley once again.
GOAL - JORDI ALBA! Game, set and match. Lightning quick break from Barcelona, and Jordi Alba gets his reward for sprinting the length of the field as he's picked out expertly by Alexis Sanchez to apply the final touch to the move, the match and the tie.
We're approaching three added minutes at the end of this match. The second half hasn't matched the first for quality, Villa's superb goal apart, but it looks like being enough for Barcelona to complete a memorable comeback.
The corner comes to nothing, but so too does any Milan break as Robinho is flagged offside.
Alves wins a corner for Barcelona that will see the clock tick round towards the final minute of the 90.
Milan pump the ball into the Barcelona penalty area, but El Shaarawy is flagged offside as he tangles with Pique.
Quick counter-attack from Barcelona, but Messi's attempted one-two with Alexis Sanchez breaks down.
Really good spell of possession for Milan in the Barca half, but there's no way through for the Italian side.
Adriano replaces Pedro as Barcelona make their final change of the night.
Nothing from the corner, and Barcelona are back in possession. Time ebbing away for Milan to find the away goal they need.
Good work from Bojan on the left, and his low cross looks sure to find Robinho only for Jordi Alba to make a superb and telling interception at the expense of a corner.
Boateng flagged offside.
Ten minutes plus stoppages for Milan to find a goal. They've offered precious little in this second half.
Puyol has replaced Mascherano for Barcelona.
Alexis Sanchez has the ball in the Milan net after being played in by Jordi Alba, but the offside flag is raised. The Barca sub tells the ref he didn't hear the whistle and avoids a booking for playing on.
The corner comes to nothing.
Iniesta's sliderule pass plays Jordi Alba in behind Milan on the left ,but Abate gets back to divert the cross behind for a corner.
Abate played in by Boateng, but the full-back hasn't held his run and is flagged offside.
First change for Barcelona as Villa makes way for Alexis Sanchez.
Bojan on against his former club in place of Flamini.
Messi hammers the free-kick into the wall and makes a mess of attempting to turn the follow-up back into the danger area.
Zapata gets too tight to Messi and fouls the Barcelona man as he looks to spin off the defender and charge in on goal. Free-kick in a really dangerous spot: 25 yards out and central.
Flag up for offside against Boateng. Milan haven't had the same sights of goal they carved out in the first half. Be a real smash and grab if they were to pinch it now.
Messi can't quite profit from a one-two with Villa on the edge of the Milan box and Abbiati hammers the ball clear.
Abate's cross from the right causes panic as it ricochets around the Barca penalty box. Chances for Robinho and Boateng, but Barca scramble the ball clear.
Robinho tries to steer the ball through for the onrushing Flamini but it's easy for Valdes.
Pedro penalised for a foul on Abate as they pursue a ball down the Barca left.
The free-kick is played beyond the far post where Jordi Alba tries to keep the ball alive but can only steer it to Abbiati.
Messi's quick feet leave Muntari floundering and almost involuntarily flicking out a leg to trip the Barca magician. Another free-kick for Barcelona.
More lovely build-up play from Barcelona ends with Messi flashing a volley over the bar.
Free-kick for Barcelona wide on the right.
Ambrosini and Niang make way for Robinho and Muntari as Milan look to change the course of the game.
A full minute of Barca possession as they patiently wait for an opening to present itself.
Intricate passing from Barca, coupled with energetic hustle and bustle to regain possession on the rare occasion a pass goes astray.
Still all is not lost for Milan if they can find an away goal from somewhere. They've had chances.
GOAL - VILLA! Barca into the lead for the first time in the tie as Villa curls home after brilliantly controlling Xavi's precise throughball.
Messi pops up on the right only to find himself upended by Mexes again. This time the Milan defender has his name taken.
Niang's corner finds Boateng, but his first-time effort never threatens the target.
Milan win corner as Jordi Alba is forced to turn the ball behind. A Milan goal changes the complexion of the game quite dramatically.
Messi threads a clever ball through for Villa, but his cutback lacks power and Milan clear.
Mexes trips the marauding Messi. Free-kick to Barca just inside Milan territory.
Hopeful ball forward aimed for Niang, but he plays the ball out of play under pressue from Mascherano.
Messi runs at the Milan goal again but after evading two challenges his shot from the edge of the box lacks the power to really trouble Abbiati.
Overhit pass aimed for Pedro goes straight out for a Milan goal-kick.
Barcelona get the second half under way.
That's the final action of a first half in which Lionel Messi has wiped out Barca's first-leg deficit. It's not all over for Milan, though. Despite being swamped by Barcelona's possession football for much of the half they've still carved out two or three very decent chances for themselves and one away goal still changes things significantly. If Milan find the target, Barca will need two more goals.
Pedro looks to launch a late Barcelona break, and is crudely chopped down by Flamini who earns a yellow card for his efforts.
Milan attacking in what should be the only added minute at the end of this first half, but Barca's backline stands firm.
Xavi, Villa and Messi can't quite link up to find a way through, and Milan have a goal-kick with time running out at the end of the first half.
Barca take the free-kick short and set about looking to pick another path through the Milan defence.
Alves does well to keep the ball in on the right and then wins a free-kick as Constant pulls him back.
For all that Messi's finish was superb again, Milan will rue not only Niang's failure to convert his one-on-one chance and also their sloppiness in midfield to allow Iniesta to pounce.
In the unlikely event that the scoreline remains this way after 90 minutes, we'll go to extra-time.
GOAL - MESSI! Huge 60 seconds in this tie. Great chance for Milan to extend their advantage and, barely a minute later, Messi is firing Barcelona level from 20 yards after Iniesta pinches possession in midfield.
What a chance for Milan! Confusion in the Barca defensive line allows Niang a clear run on Valdes' goal, but the Milan striker slides his shot past the keeper only to see it cannon back off the upright.
Milan gradually getting a foothold in proceedings after weathering that incredibly Barca storm in the opening 25 minutes.
Iniesta looks early for the run of Villa, but overhits the pass and Abbiati gathers.
Alves seeing lots of the ball, doing little with it. Loses the ball here, but recovers to win a throw.
Excellent Montolivo sliding tackle halts Messi in full flow and allows Milan to break. El Shaarawy cuts in from the left and tries to curl the ball inside Valdes' far post but scuffs his finish. For all Barca's dominance, that's two presentable chances El Shaarawy has scuffed horribly.
Barca looking to work the ball wide to Alves at every opportunity, but so far his crosses have produced little.
A Milan attack breaks down and straight away Messi is breaking away until being tripped after crossing halfway.
The good news for Milan here is that it's only 1-0. They're being totally outclassed.
Two chances for Alves to cross, but he hits the first defender both times and then fouls Constant as the Milan defender looks to bring the ball out from the back.
Flamini back in the action with a big bandage around his head.
Slick break from Barca ends with Alves' cross from the right headed well wide by Messi. Flamini, meanwhile, is receiving treatment for a cut to the head.
Milan just can't get the ball. And when they do, Barca are harrying them into mistakes. One-way traffic.
Boateng goes into the book for a clumsy foul on Iniesta.
Again, the initial set-piece comes to nothing but it only delays rathe than prevents a Barcelona attack. Pedro jinks into the penalty area before being tackled, and once again there are loud calls for a penalty. Could already have been so much worse for Milan with a different referee.
The corner comes to nothing, but Barca are soon back on the attack and win a free-kick 35 yards from goal and left of centre.
More slick Barcelona passing ends with the flustered Abate putting the ball behind for a corner.
Alves lets the ball slip under his foot and out for a throw. Rare error from Barca.
A shocking backpass from Abate almost lets in Villa, but Abbiati just does enough to keep the ball away from the Barca forward.
The corner pinballs round the Barca box before falling to Valdes.
A chance for that away goal now as Mascherano concedes a corner.
If Milan don't switch on soon, this could get ugly for them. As it is, it's already hard to remember that they actually still lead in the tie and are only an away goal from making life seriously difficult for Barca.
Close! Iniesta thunders a volley goalwards from 20 yards but Abbiati manages to claw the ball away. The ball breaks to Messi, but his header finds only the side-netting.
Huge penalty appeals from Barca as Pedro is bundled over in the penalty area by Abate as he looks to run onto Xavi's pass over the top. Nothing given.
Messi loses possession but Villa's persistence wins the ball back for Barca before the visitors can clear their lines.
Chance for Milan as El Shaarawy sneaks in behind the Barca backline but he snatches at his shot and the ball dribbles through to Valdes.
First forward foray from Milan, but Niang's progress is halted by Mascherano's strong but well-timed tackle. Camp Nou makes almost as much noise for that as it did for Messi's goal. Well, it doesn't. But it was loud.
Looks like being a long, hard night's work for Milan if they are to preserve this lead and book a quarter-final spot.
GOAL - MESSI! Brilliant, just brilliant. A typically intricate Barcelona passing move ends with Messi curling the ball into the top corner from the edge of the penalty area. Milan's advantage already cut in half.
The likely pattern for the game already established. Six Milan plaers lining up on the edge of their own box with a host of red and blue shirts running at them.
Dani Alves unhappy not to win a free-kick after being found by a long ball forward. He's playing in a very advanced role - looks like Barca have gone three at the back from the off.
Milan get us under way.
Delbhoy in Glasgow: "Would love to see Barca go through but have a feeling AC are gonna pick their pocket and it will be 2-2."
Update from the chaps at Sky Bet: "Barcelona surely must halt a run of six consecutive European home games without a clean sheet if they're to overturn a two-goal deficit against Milan. They are 2/7 to win on the night but a 6/5 chance to qualify for the quarter-finals. Milan are 15/2 to pull off a shock win at the Nou Camp but we offer 8/13 for them to go through on aggregate. Lionel Messi is 15/8 to score first. Alternatively it's 11/1 for Giampaolo Pazzini to score first and effectively end the tie."
Stu in 'the Madrid half of Spain': "Unbeaten in 10, Milan must feel confident. I think it will finish on the night 3-1 Barcelona, but enough for Milan to pass into the next round. Maybe Bojan as surprise goal scorer for Milan in 88min, ex-players always come back to haunt you!"
Sanay Patel: "Barca can turn things around they are the best team and they will score lots of goals. Prediction for tonight: Barcelona 4-0 AC Milan"
Jonathan: "Can only see this game going one way! Barcelona to win 3-0 with Messi scoring first! Away leg was a "blip" and can't see Milan having any way of stopping them this time! Looking forward to this one."
Let us know your thoughts on tonight's action, and I'll print as many as I can: Dave.Tickner@bskyb.com is the address to use. Can Barca turn things round?
Milan: Abbiati; Abate, Mexes, Zapata, Constant; Montolivo, Ambrosini, Flamini; Boateng; Niang, El Shaarawy
Barcelona: Valdes; Alves, Pique, Mascherano, Alba; Xavi, Busquets, Iniesta; Pedro, Messi, Villa
Welcome to our live coverage of the Champions League last-16 second leg between Barcelona and Milan. Barca are, by their standards, going through something of a bad run right now and face a tall order attempting to overturn a 2-0 defeat from the first leg. Team news to follow shortly.