Read how the action unfolded below.
6.47pmHello all. The Champions League - it's back! After tonight we will be at the mid point of Group F, but before that comes we have one of the clashes of the first round: Arsenal against Dortmund.
6.51pmArsenal's run - presumably - has to come to an end at some point, but there is no sign it will happen any time soon. It is currently 12 games since they were beaten, and last time in this competition they beat Napoli 2-0. Tonight will arguably be their biggest test so far.
6.55pmDortmund have gone off the boil slightly since their fine start to the season. They have won just two of their last four league games and most recently they just scraped past Hannover last weekend. However, their 3-0 win over Marseille will encourage them for tonight.
6.58pmRight, before I go any further I'll get the teams right over...
7.01pmARSENAL XI: Szczesny; Sagna, Mertesacker, Koscielny, Gibbs; Arteta, Ramsey; Rosicky, Ozil, Wilshere; Giroud
7.04pmDORTMUND XI: Weidenfeller; Grosskreutz, Subotic, Hummels, Schmelzer; Bender, Sahin; Blaszczykowski, Mkhitaryan, Reus; Lewandowski
7.07pmThe news - well, we already knew it - is that Mathieu Flamini misses out for the hosts. He will be a big miss as well. Tomas Rosicky is named in the XI in his place, but there will be some rotation with probably Aaron Ramsey alongside Mikel Arteta in the deeper midfield role.
7.11pmFor Dortmund. Well, it's a very strong lineup indeed. Mats Hummels, Marcel Schmelzer, and Jakub Blaszczykowski come in for Sokratis Papastathopoulos, Erik Durm and Pierre-Emerick Aubameyang as Jurgen Klopp makes three changes. Having Schmelzer back will be particularly important against Arsenal's attacking threat.
7.14pmToday is Arsene Wenger's 64th birthday. Can he celebrate in style with the third win from three in the Champions League? We've wished him a happy birthday on our Instagram, so check it out:
7.17pmHis special coats must have an extra two or three panels added - they really are very long. His job was under pressure last season, but Arsenal fans have forgiven and forgotten with the signing of Mesut Ozil and their fantastic start to this term.
7.21pmCan Arsenal realistically challenge for titles this year? Well, yes. The depth they have in midfield is better than the likes of Manchester United and probably even Manchester City, while Chelsea are still trying to gel. Arsenal have hit the ground running and whatever XI Wenger picks at the moment is very strong indeed.
7.26pmThe Gunners will have to be wary of Dortmund's excellent attack. It's Kuba instead of Aubameyang tonight, but Kieran Gibbs will need to be no less alert. How well the trio behind Lewandowski move will decide their impact tonight.
7.31pmAs far as the odds go Arsenal are favourites tonight, and you have to think that's about right. You can get them at 13/8 while Dortmund are around 2/1.
7.36pmSo with just about 10 to play, I suppose I should stick my neck out. I actually think this will be a draw tonight - I fancy goals in a 2-2 result. Arsenal, I think, are coming from behind here.
7.40pmFive left until we get underway! A reminder we have commentaries of all eight Champions League games tonight. Group F's other game, Marseille vs. Napoli, is right here.
7.44pmNothing quite gets you up for a night of football than that Champions League theme. And for me it's brilliant to see Dortmund back and successful - they were one of the teams I'd tune in to watch as a youngster in the 90s.
1 minKICKOFF: Arsenal, in red of course, get us underway in front of a packed Emirates crowd.
3 minLots of possession early on for Arsenal as they all try to get a touch. Dortmund close in quickly on Ozil who is almost caught in possession but wins a somewhat fortuitous free kick. The early game plan is clear for the visitors - press, harry and make Arsenal think quickly. They've given away a couple of free kicks doing so early on.
5 minFirst foray forward for Kevin Grosskreutz, but his cross from deep is woeful and goes out for a goal kick. Dortmund are seeing a bit more of the ball now, while we're yet to see anything in the way of a chance for either side.
7 minIncredibly ambitious effort from Mats Hummels. He brings the ball forward from the half way line and doesn't get closed down, but his strike from 35 yards goes flying over the bar. A bit wasteful, but in truth there was little on for the centre-back.
10 minDortmund have done a good job so far of isolating Olivier Giroud. The Frenchman hasn't had a touch yet, as Rosicky becomes the next player fouled as Subotic gets too tight to him.
12 minMkhitaryan's turn to be clattered as Arteta shunts into him from behind. The former Shakhtar man knew what was coming and was able to brace himself. At the other end Mkhitaryan tries to get between Gibbs and Koscielny, but the Frenchman squeezes him out of it. Reus picks up the ball on the right and he tries to get it into the far top corner but curls it wide. The angle was never right for the right-footer.
16 minGOAL! Arsenal 0-1 Dortmund (Henrikh Mkhitaryan)
16 minI did say I thought Arsenal would go behind and they have. It's really poor defending - first from Arteta who passed to Ramsey in a difficult spot, then the Welshman for giving it away instead of just hacking clear. Lewandowski gets the ball, slides in Mkhitaryan who puts it into the bottom corner for the lead.
19 minGiroud is almost played in, but he had drifted offside waiting for the pass. It has not been flowing so far from Arsenal - Dortmund's plan has been effective so far, as the away crown bounce in the stands.
22 minShould this result stay as it is, it will make the group incredibly exciting for the final three games. But with three sides in the top two of their respective leagues, what do you expect? Appalling tackle there from Hummels who brought down Giroud just outside the box.
23 minOzil takes the free kick which is on the right and Koscielny wins his header, but directs it over the bar. Hummels, by the way, was booked for that lunge on Giroud and rightly so.
26 minKoscielny defends well there as Dortmund were looking dangerous in the box with Kuba. However as they try to break they're quickly closed down again and the ball is back with the men in yellow.
29 minThis is not what this crowd have got used to seeing from their side so far this season. Dortmund are showing no respect and are in the face of every single red shirt on the ball. Wilshere is played in over the top by Risicky but Weidenfeller just gets there first and wins the goal kick off the midfielder. Wilshere is down after that, too.
31 minRamsey is found by Rosicky there and he has a little bit of space so goes for goal from 25 yards. There were passes on, including Wilshere to his right, but that's still more encouraging from the hosts.
34 minNot sure quite what Lewandowski is trying there. He wins the ball in midfield and he drives into the box, but does too much including a couple of attempted stepovers and in the end is penalised for a foul. Dortmund are now attacking on the break, with Arsenal seeing a lot of the ball - but not yet in dangerous areas.
37 minCLOSE! Arsenal so close to an equaliser. Initially a low cross is cleared to Rosicky, who shoots from the edge of the box but it's off the line by Hummels when he had his back turned somewhat.
39 minThis really has not lived up to its potential so far. Arsenal have not yet created a genuine chance, while the only one at the other end has gone in. Dortmund won't mind too much as they've done what they wanted to do in this half.
41 minGOAL! Arsenal 1-1 Dortmund (Olivier Giroud)
41 minRamsey does really well to twist and put in Sagna down the right. His cross is magnificent and Giroud pips Weidenfeller to it, and hammers it in after it flicked into the air. He has worked really hard in the last 10 minutes and has his reward.
44 minDortmund's defensive stability has finally been undone, but there's still a lot left in this one. What a waste from Grosskreutz, who shoots from an angle well outside the box but puts it a long, long way over.
45 minHALF TIME: Arsenal 1-1 Dortmund
8.33pmWell the half came to life in the final 10 minutes with Arsenal moving through the gears a little. They got the equaliser that they deserved, and 1-1 at the break is about right. Dortmund came out very strongly indeed.
8.36pmThe German side harried and pressed right up to the Gunners' half in the opening half an hour and it was looking for a while like Arsenal would not have an answer to it. The goal came about exactly from such pressing, with Arteta playing a risky pass to Ramsey and the Welshman then losing out.
8.40pmIn the end It was Ramsey who managed to break free of some tight pressing to find Sagna, who whipped in an unbelievable cross to Giroud. Weidenfeller might get some stick for his performance meeting the cross, but in fairness I challenge any keeper in the world to deal with that one from Sagna.
8.44pmKlopp won't be too disappointed with that half - from his view in the stands, of course - and nor will Wenger. Both sides had spells of dominance and scored in them. It hasn't been particularly thrilling from either team in attack, but it's really well poised.
46 minKICKOFF: Dortmund get us back underway for the second 45.
48 minDortmund have started well in this half and have taken the early control of possession. They are all getting a touch at the moment, including Subotic who has marauded forward into an attacking position. Patience for now from the Germans.
51 minBOOKING: A bit of frustration to the early Dortmund possession by Rosicky, who brought down Mkhitaryan. It wasn't the worst tackle you'll see in the game but still he goes into the book.
53 minReus makes a run for Grosskreutz to find him, but the pass is quickly smothered by a couple of red shirts. The Germany international has been quiet so far - but he is of course the type of player who can change the game in an instant.
55 minGiroud tracks Reus all the way back and then finds Wilshere, who finds himself the furthest man forward. In the end they win a throw following Subotic's overhit backpass that Weidenfeller manages to stop going for a corner.
56 minHalf-chance there for Grosskreutz, who again has got right up the field from right-back. He's not sure whether to head of shoot and in the end tamely passes it to Szczesny in the Arsenal goal.
58 minARSENAL SUB: Santi Cazorla is not a bad player to be able to bring on is he?! He takes to the field in place of Wilshere.
61 minLovely football from Arsenal. Ozil plays to Giroud, who touches it back to him with one touch as the German continues his march into the box. In the end Dortmund defend well and concede a corner, but Arsenal cannot work a chance from it. They're just about on top at the moment, however.
64 minBOOKING: Cazorla has been bright since coming on. He's played wide left by Gibbs, but his cross is just too close to Weidenfeller. Lewandowski then elbows Koscielny in the face! It's not a swing but he knew what he was doing - the referee immediately books him but if he's seen that as an elbow it's surely red?!
66 minDORTMUND SUB: The rapid Pierre-Emerick Aubameyang is on for Kuba. The Pole didn't have a lot of joy - can the summer signing? Gibbs is about to have to start running very hard to keep up with him I suspect.
69 minCLOSE! Great effort from Cazorla on his weaker foot. Arsenal shift through the gears and the ball is cut back to him by Ozil but his shot from just outside the box bends and clips the angle before going over.
71 minThe Spaniard has really taken this game by the scruff of the neck since coming on. His teammates have followed suit and are now pressing really high up the pitch. They sense a winner here and the crowd are responding, too.
74 minIn the other game in the group, Napoli are now 2-0 up away to Marseille. That's a big result for the Italians should they hang on and will really open up the group. Of course, should Dortmund win here they would all be on six points.
76 minIt's still the Cazorla show here. He takes a lovely touch in the box from Ozil's pass, before cutting inside two along the length of the box. His effort is blocked, however, and Arsenal go back to Szczesny afterwards. This half has certainly been Arsenal's - can they capitalise.
79 minMore pressure from Arsenal. Sagna crosses from the right, but Giroud doesn't make the contact he wants as he had to check his run slightly. It comes nearly through to Cazorla, but Weidenfeller is out quick to gather.
82 minGOAL! Arsenal 1-2 Dortmund (Robert Lewandowski)
82 minThat is a real sucker punch for Arsenal. First it's good work again from Sagna, who takes a good first touch before working space for a cross. However, there's no-one in the box waiting. Dortmund break through again Grosskreutz down the right and there are more yellow than red shirts in the box for the cross. It falls to the one you want, as Lewandowski guides a low volley into the net. He probably shouldn't even be on the pitch still.
85 minBOOKING: Ozil is booked and probably for his reaction. Hummels gets forward to just to the left of the box and Ozil brings him down - there was a slight touch on the ball, but a silly tackle even so.
86 minSUBS: Nicklas Bendtner comes on for Ramsey for the hosts as Wenger rolls the dice for a final time. Meanwhile it's the opposite change for Dortmund as Sokratis comes on for Reus.
89 minThe goal has knocked the wind out of Arsenal and I don't think they will recover in time to get the leveller. It's looking to be a very harsh result for Arsenal and from two opening wins it suddenly looks a very tricky route into the next round.
90 minARSENAL SUB: Serge Gnabry is on for the last few minutes.
90+1 minDortmund have now taken it to the corner and it looks likely to stay there for the final two minutes of stoppage time.
90+3 minAs Arsenal look for a late attack they're once more crowded out in midfield. They come again through Gnabry, but his cross is cut out by Subotic.
90+3 minFULL TIME: Arsenal 1-2 Borussia Dortmund
9.36pmSo there we have it. It's a killer blow for Arsenal who lose to a late winner from Lewandowski, who might have been sent off earlier. The group is now level with Arsenal, Dortmund and Napoli all on six points.
9.37pmThat's all from me for this one. We'll have all the reports and reaction on site this evening, as well as analysis from this and the Chelsea game. Look out for those within half an hour. Until next time, goodbye.