Arsenal, Chelsea, Frankfurt and Valencia have made it to the last four; find out all about them.
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Arsenal v Valencia
Frankfurt v Chelsea
UEFA ranking: 9
This season: W9 D1 L2 F22 A6 (entry: group stage)
Knockouts: 3-1 v BATE Borisov, 4-3 v Rennes, 3-0 v Napoli
Last season: UEFA Europa League semi-finals
UEFA Cup/UEFA Europa League best: runners-up (2000)
Season in a nutshell: Arsenal's first season without Arsène Wenger since 1995/96 has been an enigmatic affair. Eleven successive wins between August and October had fans singing "We've got our Arsenal back", but since then the club's familiar defensive issues have plagued their progress. Nevertheless, the goal remains a UEFA Champions League return – either as UEFA Europa League winners or top-four finishers in England.
Key player: Pierre-Emerick Aubameyang. Arsenal's top scorer has also been a creator for the team, and he made a huge contribution to the comeback against Rennes which fired the team's challenge in the knockout stages.
Why this is their year: Unai Emery. Arsenal have never shifted their focus from the UEFA Europa League this season, and it's unsurprising. This is a man who knows the tournament backwards and has made The Gunners even more competitive in it.
UEFA ranking: 13
This season: W11 D1 L0 F30 A7 (group stage)
Knockouts: 5-1 v Malmö, 8-0 v Dynamo Kyiv, 5-3 v Slavia Praha
Last season: UEFA Champions League round of 16
UEFA Cup/UEFA Europa League best: winners (2013)
Season in a nutshell: Chelsea kicked off with five straight victories in the Premier League as former Napoli coach Maurizio Sarri settled in, yet that early momentum gradually began to fade. Although the Blues finished runners-up in the English League Cup, they have slipped in the top-flight standings and suffered FA Cup elimination – increasing the onus on their UEFA Europa League tilt.
Key player: Olivier Giroud. The UEFA Europa League's top scorer, the Frenchman's ten goals have been pivotal in Chelsea's run to the semi-finals. The world champion leads the line powerfully, links well with the wide players and has an eye for the spectacular.
Why this is their year: Experience. The Blues have lifted a trophy in each of the last two seasons and reached a domestic cup final as recently as February. César Azpilicueta, David Luiz, Gary Cahill and Eden Hazard remain from the squad that won this competition in 2013.
UEFA ranking: 39
This season: W5 D1 L0 F11 A3 (round of 32)
Knockouts: 3-0 v Celtic, 3-2 v Krasnodar, 5-1 v Villarreal
Last season: N/A (previous campaign: 2015/16)
UEFA Cup/UEFA Europa League best: winners (2004)
Season in a nutshell: The only team that transferred into the competition from the UEFA Champions League who are still in the running, Valencia began the season with just one win from their first 13 games in all competitions. Since then, they have steadily climbed the Liga table while booking themselves a spot in the Copa del Rey final against Barcelona in May.
Key player: Dani Parejo. There is a terrific supporting cast across the Valencia squad but the captain is the leader, the artist, the old-hand and the calm talent. If storms lie ahead he'll steer them through.
Why this is their year: Momentum. It's the centenary of this fine club and while they've been difficult to beat all season, in recent weeks they've been on a roll – quick, lots of conviction, the goals flowing and oozing belief. Not just their 100th year; THEIR year.
UEFA ranking: 68
This season: W9 D2 L1 F28 A12 (group stage)
Knockouts: 6-3 v Shakhtar Donetsk, 1-0 v Internazionale, 4-4 v Benfica (won on away goals)
Last season: N/A (previous campaign: 2013/14)
UEFA Cup/UEFA Europa League best: winners (1980)
Season in a nutshell: What a turnaround it has been since the start of the campaign, when Frankfurt lost four of their first five matches under Adi Hütter. The successor to Niko Kovač held his nerve and has guided the club from 15th in the Bundesliga to top-four contention, while regularly impressing in Europe – thanks largely to prolific duo Luka Jović and Sébastien Haller.
Key player: Luka Jović. After showing glimpses of his qualities last season, the Serbia striker has proven his pedigree and taken a giant stride forward in his development. When he's on form, Frankfurt are at their most dangerous.
Why this is their year: Frankfurt are riding a wave of euphoria that stems from ending a 30-year wait for silverware last season. With big clubs eyeing their key players, this is their best chance at adding to the UEFA Cup they won in 1980.