Monaco are showboating now as Joao Moutinho cleverly flicks the ball with the back of his foot into the path of Anthony Martial, who is racing into the Arsenal area, he has a clear sight of goal but David Ospina is quick off his line to close down the gap and deflect the ball behind.

Former Arsenal and England centre-back Martin Keown
5 live Sport

"Monaco have been perfect in their performance. Their defence is resolute and their midfield has been very mobile. They are mopping everything up in the middle of the park."

Image source, AP

Olivier Giroud pays the price for missing a number of chances as he is replaced by Theo Walcott. Arsenal lose the focal point of their attacks, but gain some welcome pace.

Former Arsenal and England centre-back Martin Keown
5 live Sport

"In all honesty you have to feel for Giroud, because that was a tap-in. It landed on his right foot, unfortunately for him, and he skied it. I'd consider taking him off."

Oh my word. What a miss by Oliver Giroud as Alexis Sanchez's drive is parried into the path of the striker by Danijel Subasic and it should be 2-1, but he somehow smashes high over with the goal at his mercy. This is just not going to be Arsenal's night.

Phil McNulty
BBC Sport chief football writer at Emirates Stadium

Image source, AFP

"We've seen it before and now we've seen it again. Arsenal have been caught out hopelessly upfield on the break as Monaco sweep forward and Dimitar Berbatov adds the second. Absolutely criminal to be undone like that at this stage of the game - hardly panic stations yet but so many in Monaco's half and barely anyone minding the shop at the back. What a battle Arsenal face now."

Samuel Harper: There's always the FA Cup, Arsenal...

Will Brookes: At least if it was Munich or Madrid you could accept the humiliation. And so our annual February collapse begins...

Jonny Walker: I think that Arsenal have underestimated their opponents tonight, what with all the talk that they were lucky with the draw.

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Former Arsenal and England centre-back Martin Keown
5 live Sport

Image source, Reuters

"What a move from Monaco on the break. A lovely ball into Berbatov's path and he thunders his effort into the back of the net. He may drive to training in a Ferrari and go home to his deck chair but he was purring across the ground there. I take it all back - he's punished Arsenal tonight."

Image source, Reuters

Shocking, shock defending by Arsenal as Monaco double their lead and it has to be that man Dimitar Berbartov. It comes after an Arsenal attack breaks down and Monaco counter quickly. Per Mertesacker ventures forward and leaves Anthony Martial in acres of space and he passes to Berbatov, who fires into the top corner. What was Mertesacker doing?

Mungo Russell: Games like these that show why Arsenal haven't won the premier league in over 10 years. No steel, too many players out the game.

Matt Williams: Not enough credit is being given to Monaco. Unsettled the crowd, solid defence, grabbed a goal. The perfect away performance.

Fore Left: People forget Monaco have let one Champions League goal in this season and won their group. This is a tough game.

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The crowd respond to Arsenal's increase in tempo with a few chants of "come on Arsenal". There are chances to be had if Arsenal run at the Monaco defence, which Hector Bellerin does to get away down the right, but he plays the ball behind Olivier Giroud inside the away area.

Former Arsenal and England centre-back Martin Keown
5 live Sport

Image source, PA

"Giroud really should be doing better - the ball has hit his standing foot. There is a look of anguish on his face. He knows he should be doing better."

This is what Arsenal fans want to see! The Gunners are straight on the offensive as Alexis Sanchez breezes forward, weaving his way through the Monaco defence before cutting the ball back to Olivier Giroud at the near post, but he pokes his shot just wide.

Just looking back over Monaco's results this season and I make it they have conceded just two goals in their last 13 games in all competitions. Tough task ahead for Arsenal.

Can they respond? The second half is under way.

Former Arsenal and England centre-back Martin Keown
5 live Sport

"Ozil has not quite been in the game and Welbeck has drifted out. The Monaco goal was just a lucky strike, a punt from 30 yards. But I think Monaco deserve it because of the quality they've shown defensively. Monaco will now be buzzing in the second half, but they know Arsenal will come at them."

Image source, OptaJoe

Dimitar Berbatov may be Monaco's top goalscorer but he has not looked likely to add to his seven goals tonight, having never got near the Arsenal area in the first half.

In fact, the only Monaco player to have touched the ball in the Gunners' box was Joao Mountiho, with their only shot on target Geoffrey Kondogbia's long range strike. That resulted in the opening goal, though, and that is all that matters as far as the visitors are concerned.

James: As a Manchester United fan, although we're playing badly right now, you only have to watch Arsenal to feel better...

Eoin from London: So far, really poor from Arsenal. Very wasteful. Need more pace, and I think the Gods are calling for Walcott to be on that pitch. It has not been Welbeck's night tonight.

Pat in Glasgow: Arsenal are not genuine Champions League contenders because they cannot play ugly and win, or play poorly and win.

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Forest have clawed their way back into this game, coming from a goal down to lead 2-1 at the interval, thanks to a strike almost on the whistle from Henri Lansbury.

The hosts have won three of their last four games, drawing one. Their last defeat was 1-0 against Millwall at home back on 31 January.

If Arsenal really are to go all the way in this year's Champions League, they are going to have to quickly improve their defensive frailties. They have kept only two clean sheets in their last eight Champions League knockout games at home.

Phil McNulty
BBC Sport chief football writer at Emirates Stadium

"Grim first half that one for Arsenal. Barely caused Monaco a moment's trouble - no tempo to their play and they have never looked like piercing a very solid defence. More urgency needed after the break."