What is each club's pedigree? Who are their key players, new faces and ones to watch? UEFA.com has all the answers.
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UEFA coefficient ranking (end of 2018/19): 12
How they qualified: UEFA Europa League winners
European Cup best: winners (2012)
Last season: #UEL winners (beat Arsenal 4-1 in the final)
In: Mateo Kovačić (Real Madrid)
Out: Eden Hazard (Real Madrid), Gary Cahill (released), David Luiz (Arsenal), Davide Zappacosta (Roma, loan)
Coach: Frank Lampard
Revered as a player following his move from West Ham, Lampard spent 13 seasons at Stamford Bridge and won the UEFA Champions League, UEFA Europa League, three league titles and four FA Cups. Made his managerial bow last season at Derby County, reaching the second-tier play-off final, before Chelsea came calling in the summer.
Key player: N'Golo Kanté
The industrious central midfielder was the catalyst for Leicester's fairy-tale Premier League triumph in 2016 and had the same effect at Chelsea as they won the title the following season. Has won a trophy in each season since and will be expected to marshal the new breed of young talent at the club this season.
One to watch: Mason Mount
Callum Hudson-Odoi and Ruben Loftus-Cheek both shone during Chelsea's run to European success last term and 20-year-old Mount looks to be the next off the conveyor belt. Lampard knows him well having taken him on loan to Derby last season, where he scored eight times on their way to the play-off final, and the departure of Eden Hazard could work to the talented midfielder's advantage.
UEFA coefficient ranking (end of 2018/19): 20
How they qualified: Dutch champions, play-off winners (beat APOEL 2-0 on aggregate)
European Cup best: winners x4 (1971, 1972, 1973, 1995)
Last season: semi-finals (lost on away goals, 3-3 on aggregate, to Tottenham)
Group stage last season: Group E runners-up – W3 D3 L0 F11 A5 (v Bayern, Benfica, AEK Athens)
In: Quincy Promes (Sevilla), Edson Álvarez (Club América), Răzvan Marin (Standard Liège)
Out: Frenkie de Jong (Barcelona), Matthijs de Ligt (Juventus), Lasse Schöne (Genoa), Kasper Dolberg (Nice)
Coach: Erik ten Hag
A defensive midfielder and defender, his playing career took in Twente (three times), De Graafschap, RKC Waalwijk and Utrecht. Took his first post as head coach at Go Ahead Eagles, winning promotion to the Dutch top flight in his 2012/13 debut season, before moving on to Bayern München's second team. Took over at Utrecht in summer 2015, winning the Rinus Michels award as the Netherlands' coach of the year 12 months later. Succeeded Marcel Keizer at Ajax in December 2017, won the domestic double last season and led his side to the UEFA Europa League last four.
Key player: Dušan Tadić
One half of Ajax's creative tandem with Hakim Ziyech, Tadić contributed six UEFA Champions League goals and a further five assists last time around – fine numbers for a player who only joined from Southampton at the start of the season. The captain ended the campaign with a remarkable 38 goals in 56 games in all competitions.
One to watch: Carel Eiting
Frenkie de Jong's potential successor with the Dutch giants, the 21-year-old has an unorthodox style coupled with vision and intelligence. Featured heavily in qualifying for the competition last season and played 90 minutes in the group stage against AEK Athens.
UEFA coefficient ranking (end of 2018/19): 39
How they qualified: fourth in Spanish Liga
European Cup best: runners-up x2 (2000, 2001)
Last season: group stage (third place), #UEL semi-finals
Group stage last season: third in Group H – W2 D2 L2 F6 A6 (v Juventus, Manchester United, Young Boys)
In: Jasper Cillessen (Barcelona), Maxi Gómez (Celta Vigo), Eliaquim Mangala (Manchester City)
Out: Neto (Barcelona), Jeison Murillo (Sampdoria), Santi Mina (Celta Vigo)
Coach: Albert Celades
Appointed on 11 September to replace Marcelino, Valencia is a first senior managerial role for the former defensive midfielder. Celades emerged at Barcelona, winning two Liga titles and the 1997 UEFA Cup Winners' Cup. He won two more league crowns at Real Madrid as well as a 2002 UEFA Champions League winner's medal. He replaced Julen Lopetegui as Spain Under-21 coach in 2014 and later worked as assistant to Lopetegui at Real Madrid.
Key player: Daniel Parejo
The midfield all-rounder joined the club in summer 2011 and has been the heartbeat of the side since. He made 55 appearances in all competitions last season, getting into double figures for goals for the third time in five years.
One to watch: Ferran Torres
The second player born in 2000 – after Vinícius Júnior – to score in the Spanish Liga, Torres also hit both goals in Spain's 2019 Under-19 EURO final win against Portugal. The 19-year-old has already passed 50 appearances for the club.
UEFA coefficient ranking (end of 2018/19): 114
How they qualified: runners-up in French Ligue 1
European Cup best: round of 16 (2007)
Last season: did not play European football
In: Timothy Weah (Paris), Victor Osimhen (Charleroi), Renato Sanches (Bayern)
Out: Thiago Mendes (Lyon), Rafael Leão (AC Milan), Nicolas Pépé (Arsenal)
Coach: Christophe Galtier
Played for Marseille (twice), LOSC, Toulouse, Angers, Nîmes and Monza before spending a decade in various assistant coach roles. Finally took sole command with St-Étienne in 2009, where he remained until summer 2017, having picked up a Coupe de la Ligue four years earlier.
Key player: José Fonte
The 35-year-old's experience is vital to a side brimming with young talent. Still at the peak of his powers, the dominant centre-back was key to LOSC finishing second in his first season at the club and capped that campaign with UEFA Nations League glory with Portugal. Also a winner at UEFA EURO 2016.
One to watch: Jonathan Ikoné
The former Paris prodigy played every Ligue 1 game for LOSC after joining on a five-year deal last summer. An Under-17 EURO winner in 2015, the attacking midfielder impressed for France at this summer's Under-21 event too.
Did you know?
- Chelsea have got out of their UEFA Champions League group on 14 of their previous 15 participations – the sole occasion they didn't, in 2012/13, they became the first and so far only holders to fail to reach the knockout rounds.
- Ajax are in the UEFA Champions League for the 15th season. In 2018/19, they became the first team to reach the semi-finals having come through three qualifying rounds.
- Chelsea's most recent meetings with Valencia came in the group stage of their victorious 2011/12 UEFA Champions League campaign (1-1 a, 3-0 h).
- While LOSC have never played Chelsea or Ajax, they have lost three of their four matches against Valencia (D1).
- Ajax and Valencia's only two meetings came in the 2002/03 second group stage. Both games were drawn with both sides eventually progressing to the quarter-finals.