Find out who the Gunners get in tomorrow's draw at 11:00 BST. That will be followed by the Champions League draw. Follow both of them on the BBC Sport website.
Wenger hopes to avoid Atletico
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal
Arsenal manager Arsene Wenger on BT Sport...
On the performance: "I feel that maybe we were a bit surprised by the intensity at the start.
"After that we came back. They scored the second, had a chance for the third. It was a good response from us after the break, the first half the distances were too big. At 2-1 we looked good to get to 2-2 and we did.
"We were in trouble in the first half. After moving to a back five we were more
stable, didn't concede as many chances. Their second was from distance, but in the box we didn’t give anything away."
On their goals: "We have players who can always score and once we got
to 2-1 it was a big blow for them and they didn’t recover."
On reaching a European semi-final: "It should be every year, never enough, could see in Europe a
lot of turnovers, we need to be focused. Let’s prepare well for the next round and see who we get."
Want to avoid Atletico?: "Ideally, maybe they are the strongest on paper, but let's play well against the team we get."
Post update
Marseille 5-2 RB Leipzig (5-3 agg)
Just seen a clip of Marseille's injury-time clincher. RB Leipzig brought their keeper up for a corner, which they then played short.
Marseille broke and Hiroki Sakai shot from 35 yards into an empty net. Scenes.
Post update
Arsenal boss Arsene Wenger has just been chatting on the TV, we'll let you know what he's said in a minute.
Get Involved
#bbcfootball
Gregory Mandapat: I would hope for Arsenal and Salzburg in the Europa League semi-finals, but what they did to Lazio today, I am not so sure. Should be an exciting draw tomorrow.
Abdul Said: So if Arsenal win the Europa League,will Arsene W. stay because he will claim they are strong now or he will bow down in style as a legend?
Ese Agboaye: I always knew Arsenal would make it through. ALWAYS.
Post update
Red Bull Salzburg 4-1 Lazio (6-5 agg)
Red Bull Salzburg scored three goals within 247 seconds, setting a new Europa League record.
This week's games have been like a glitch in the Matrix.
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
CSKA Moscow are now winless in eight European matches against
English opposition (D2 L6). They were seconds away from ending that run tonight.
'It was not the plan'
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal captain Laurent Koscielny to BT Sport...
On a hard-fought result: "It was not the plan. I
think we missed a lot of movement, to be available for our team-mates. We didn’t
win duels. They played well, had opportunities and took them. It put pressure on
us, if they score we are out. So I am happy because we came back, but our
performance was not enough for the stage of the competition.
"We will see the draw,
but need more confidence in our game."
On reaching a European semi-final after many years: "No pressure. It is important
for the club to go far. Today we qualified so I am happy. It is our last chance
to qualify for the Champions League, so we will fight until the end."
Get Involved
#bbcfootball or 81111 on text
Kenni: Arsenal sure had its own idea of a comeback.
Ankur: Never in doubt.
David: Never thought Arsenal would lose but why do they make it so difficult? Some of us are into our 70's and don't need this stress.
'We stuck through it'
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal
Arsenal striker Danny Welbeck on BT Sport...
On the win: "It wasn’t comfortable, we have seen
in Europe how difficult it can be coming into games with an advantage. It was difficult today, away from home in different conditions, but we stuck through it."
On his goal: "The defender was tight
to me, he thought I would go inside like before, I went to the other side, played the one-two with Mo [Elneny], knew it would beat the defender and finished it nicely."
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal are the fifth English team to reach the semi-finals of the Europa League, after Fulham (2009-10), Liverpool (2009-10 and 2015-16), Chelsea (2012-13) and Man Utd (2016-17).
That's only since its 2009 rebrand from Uefa Cup, mind.
It's Arsenal's first European semi-final since 2008-09, when
they did so in the Champions League.
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
Mohamed Elneny is the first Egyptian player with two assists in a European game since Mohamed Salah in November 2012 (for Basel v Sporting in the Europa League).
Post update
Over the four ties, we've had a 6-5 and two 6-3 aggregate scorelines. Only Atletico's 2-1 win over Sporting let us down for goals.
Get Involved
#bbcfootball or 81111 on text
Prendie: Ramsey. Tick! Home and dry! Semi final next.
Dave: Ramsey and Arsenal not pumpkins now, they shall now go to the ball afterall.
Karina: It is not how you start, but how you finish is what counts.
Post update
That means no all-Red Bull clash in the semis or final, to be honest it looked like Salzburg were going out and Leipzig were going through.
But on this crazy week where down is up and left is right, it all changed.
FULL-TIME
Marseille 5-2 RB Leipzig (5-3 agg)
A Dimitri Payet-inspired Marseille battered RB Leipzig to complete a comeback of their own.
From 2-0 down on aggregate after two minutes, they turned it around to win 5-1 on the night and sail through to the last four.
ReutersCopyright: Reuters
FULL-TIME
Sporting Lisbon 1-0 Atletico Madrid (1-2 agg)
It's all over in Lisbon. Two-time champions, and current favourites, Atletico Madrid are into to the semi-finals, but they were made to work for it in Portugal.
ReutersCopyright: Reuters
FULL-TIME
Red Bull Salzburg 4-1 Lazio (6-5 agg)
There had to at least one monumental comeback. Red Bull Salzburg are through to the Europa League semi-finals.
Trailing 4-2 from in the first leg at Lazio they won 4-1 on the night thanks to a whirlwind four minutes in the second half which saw the Austrian side score three goals.
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
The scoreline makes it look fairly comfortable - but it really wasn't. Arsenal are a tad fortunate to not lose the second leg of this game.
FULL-TIME
CSKA Moscow 2-2 Arsenal (3-6 agg)
It's all over. Arsenal are into the Europa League semi-finals.
Live Reporting
Emlyn Begley
All times stated are UK
Get involved
ReutersCopyright: Reuters 
ReutersCopyright: Reuters
Latest PostGoodnight
Thanks for joining us on another night of comebacks.
For a while we thought it would be CSKA Moscow against Arsenal, but the Gunners escaped in the end.
It was Red Bull Salzburg's night.
The four Europa League semi-finalists are Arsenal, Atletico Madrid, Salzburg and Marseille.
Check out the Arsenal report here.
Find out who the Gunners get in tomorrow's draw at 11:00 BST. That will be followed by the Champions League draw. Follow both of them on the BBC Sport website.
Wenger hopes to avoid Atletico
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal
Arsenal manager Arsene Wenger on BT Sport...
On the performance: "I feel that maybe we were a bit surprised by the intensity at the start.
"After that we came back. They scored the second, had a chance for the third. It was a good response from us after the break, the first half the distances were too big. At 2-1 we looked good to get to 2-2 and we did.
"We were in trouble in the first half. After moving to a back five we were more stable, didn't concede as many chances. Their second was from distance, but in the box we didn’t give anything away."
On their goals: "We have players who can always score and once we got to 2-1 it was a big blow for them and they didn’t recover."
On reaching a European semi-final: "It should be every year, never enough, could see in Europe a lot of turnovers, we need to be focused. Let’s prepare well for the next round and see who we get."
Want to avoid Atletico?: "Ideally, maybe they are the strongest on paper, but let's play well against the team we get."
Post update
Marseille 5-2 RB Leipzig (5-3 agg)
Just seen a clip of Marseille's injury-time clincher. RB Leipzig brought their keeper up for a corner, which they then played short.
Marseille broke and Hiroki Sakai shot from 35 yards into an empty net. Scenes.
Post update
Arsenal boss Arsene Wenger has just been chatting on the TV, we'll let you know what he's said in a minute.
Get Involved
#bbcfootball
Gregory Mandapat: I would hope for Arsenal and Salzburg in the Europa League semi-finals, but what they did to Lazio today, I am not so sure. Should be an exciting draw tomorrow.
Abdul Said: So if Arsenal win the Europa League,will Arsene W. stay because he will claim they are strong now or he will bow down in style as a legend?
Ese Agboaye: I always knew Arsenal would make it through. ALWAYS.
Post update
Red Bull Salzburg 4-1 Lazio (6-5 agg)
Red Bull Salzburg scored three goals within 247 seconds, setting a new Europa League record.
This week's games have been like a glitch in the Matrix.
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
CSKA Moscow are now winless in eight European matches against English opposition (D2 L6). They were seconds away from ending that run tonight.
'It was not the plan'
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal captain Laurent Koscielny to BT Sport...
On a hard-fought result: "It was not the plan. I think we missed a lot of movement, to be available for our team-mates. We didn’t win duels. They played well, had opportunities and took them. It put pressure on us, if they score we are out. So I am happy because we came back, but our performance was not enough for the stage of the competition.
"We will see the draw, but need more confidence in our game."
On reaching a European semi-final after many years: "No pressure. It is important for the club to go far. Today we qualified so I am happy. It is our last chance to qualify for the Champions League, so we will fight until the end."
Get Involved
#bbcfootball or 81111 on text
Kenni: Arsenal sure had its own idea of a comeback.
Ankur: Never in doubt.
David: Never thought Arsenal would lose but why do they make it so difficult? Some of us are into our 70's and don't need this stress.
'We stuck through it'
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal
Arsenal striker Danny Welbeck on BT Sport...
On the win: "It wasn’t comfortable, we have seen in Europe how difficult it can be coming into games with an advantage. It was difficult today, away from home in different conditions, but we stuck through it."
On his goal: "The defender was tight to me, he thought I would go inside like before, I went to the other side, played the one-two with Mo [Elneny], knew it would beat the defender and finished it nicely."
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
Arsenal are the fifth English team to reach the semi-finals of the Europa League, after Fulham (2009-10), Liverpool (2009-10 and 2015-16), Chelsea (2012-13) and Man Utd (2016-17).
That's only since its 2009 rebrand from Uefa Cup, mind.
It's Arsenal's first European semi-final since 2008-09, when they did so in the Champions League.
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
Mohamed Elneny is the first Egyptian player with two assists in a European game since Mohamed Salah in November 2012 (for Basel v Sporting in the Europa League).
Post update
Over the four ties, we've had a 6-5 and two 6-3 aggregate scorelines. Only Atletico's 2-1 win over Sporting let us down for goals.
Get Involved
#bbcfootball or 81111 on text
Prendie: Ramsey. Tick! Home and dry! Semi final next.
Dave: Ramsey and Arsenal not pumpkins now, they shall now go to the ball afterall.
Karina: It is not how you start, but how you finish is what counts.
Post update
That means no all-Red Bull clash in the semis or final, to be honest it looked like Salzburg were going out and Leipzig were going through.
But on this crazy week where down is up and left is right, it all changed.
FULL-TIME
Marseille 5-2 RB Leipzig (5-3 agg)
A Dimitri Payet-inspired Marseille battered RB Leipzig to complete a comeback of their own.
From 2-0 down on aggregate after two minutes, they turned it around to win 5-1 on the night and sail through to the last four.
FULL-TIME
Sporting Lisbon 1-0 Atletico Madrid (1-2 agg)
It's all over in Lisbon. Two-time champions, and current favourites, Atletico Madrid are into to the semi-finals, but they were made to work for it in Portugal.
FULL-TIME
Red Bull Salzburg 4-1 Lazio (6-5 agg)
There had to at least one monumental comeback. Red Bull Salzburg are through to the Europa League semi-finals.
Trailing 4-2 from in the first leg at Lazio they won 4-1 on the night thanks to a whirlwind four minutes in the second half which saw the Austrian side score three goals.
Post update
CSKA Moscow 2-2 Arsenal (3-6 agg)
The scoreline makes it look fairly comfortable - but it really wasn't. Arsenal are a tad fortunate to not lose the second leg of this game.
FULL-TIME
CSKA Moscow 2-2 Arsenal (3-6 agg)
It's all over. Arsenal are into the Europa League semi-finals.